Science: Science is a rigorous endeavor that organizes knowledge through testable explanations and predictions about the world. It is divided into natural sciences (physical world), social sciences (individuals and societies), and formal sciences (formal systems). Some debate exists about whether formal sciences are scientific disciplines, as they lack empirical evidence. Applied sciences, such as engineering and medicine, utilize scientific knowledge for practical purposes.

Experiment: An experiment is a procedure used to investigate a hypothesis and assess the effectiveness or probability of something new. It reveals cause-and-effect relationships by manipulating specific factors to observe outcomes. Experiments range in goals and size but maintain a repeatable process and logical analysis of results. It is worth mentioning that there are also natural experimental studies.

Nature: Nature is the fundamental character of the ecosphere and the universe. It encompasses the laws, elements, and phenomena of the physical world, including life. While humans are part of nature, their activities can sometimes conflict with or be seen as separate from nature.

Observation: Observation is the act of perceiving and gathering information from the environment. It can be done by using the senses or scientific instruments. Observations in science can be qualitative or quantitative, depending on whether properties are noted or measured. The term also refers to data collected during scientific activities.

Research: Research is systematic work to increase knowledge, involving collecting and analyzing evidence to understand a topic while controlling for biases. It may build on previous work and validate instruments or experiments by replication.

Field research: Field research is the gathering of data outside controlled environments. It includes observing animal behavior in their habitats or studying people in natural settings to understand their language, culture, and social systems. Varies across disciplines.

Laboratory: A laboratory is a controlled facility for scientific research and experiments, commonly found in schools, universities, research institutions, and various other settings. It provides optimal conditions for conducting measurements and technological investigations. Laboratories can also be found in medical establishments, regulatory centers, and even private residences at times.

Scientific law: Scientific laws are statements describing natural phenomena based on repeated experiments and observations. They can predict outcomes and are developed from empirical evidence. Laws reflect fundamental causal relationships and are discovered rather than invented.

Scientific method: The scientific method is a systematic way of acquiring knowledge through careful observation, skepticism, hypothesis formulation, testability, experimentation, and refinement. It has been crucial in the development of science since the 17th century, allowing for unbiased interpretation of observations.

Scientific modelling: Scientific modelling is the creation of models that represent real-world objects, phenomena, or processes to enhance understanding. This involves selecting relevant aspects of the situation and developing a model to replicate the system. Different types of models are used for different purposes, such as conceptual models for understanding, mathematical models for quantification, computational models for simulation, and graphical models for visualization.

Scientific theory: A scientific theory is a repeatedly tested and corroborated explanation of the natural world and universe. It is evaluated using the scientific method, observation, measurement, and evaluation. Controlled experiments are conducted where possible, or abductive reasoning is used. Established theories have withstood rigorous scrutiny and represent scientific knowledge.

System: A system is a unified whole consisting of multiple elements that interact with each other based on rules. It is influenced by its environment and characterized by its boundaries, structure, and purpose. Systems are studied in systems theory and other systems sciences.

Cybernetics: Cybernetics is a systems science that studies circular causal systems with feedback. It focuses on circular causal processes in various systems, including ecology, technology, biology, cognition, and society. It also applies to practical activities like design, learning, and management.

Measurement: Measurement is the quantification of attributes for comparison with other objects or events. It determines the size of a physical quantity using a reference point. Its scope varies depending on the context and discipline. In natural sciences and engineering, measurements don't apply to nominal properties. However, in fields like statistics and social sciences, measurements can have different levels, including nominal, ordinal, interval, and ratio scales.

Accuracy and precision: Accuracy and precision are measures of observational error. Accuracy refers to how close measurements are to the true value, while precision refers to how close the measurements are to each other.

Metrology: Metrology is the study of measurement, crucial in linking human activities. It originated from the French Revolution's desire to standardize units, resulting in the creation of the metric system in 1795. The Bureau International des Poids et Mesures (BIPM) was established to ensure conformity among countries adopting the metric system. This led to the development of the International System of Units (SI) in 1960.

Unit of measurement: A unit of measurement is a fixed quantity used as a standard for measuring the same kind of quantities. Any other quantity of that kind can be expressed as a multiple of this unit.

Imperial units: The term 'Imperial units' refers to the system of units established by the British in 1824. It was further developed through subsequent laws.

United States customary units: United States customary units are a system of measurement units used in the United States and U.S. territories since 1832. They evolved from British units before the U.S. gained independence. Although similar to the imperial system, there are notable differences between the two systems.

Metric system: The metric system is a decimal system of measurement. The International System of Units (SI) is the current international standard for the metric system. It consists of seven base units: meter, kilogram, second, ampere, Kelvin, mole, and candela.

International System of Units: The International System of Units (SI) is a widely used system of measurement, coordinated by the International Bureau of Weights and Measures. It is the modern form of the metric system and has official status in nearly every country worldwide. SI is employed in science, technology, industry, and everyday commerce.

Natural units: Natural units in physics are a system of measurement where only universal physical constants are used as defining units. Each constant functions as a coherent unit for a specific quantity, such as the elementary charge for electric charge. This system defines all units as combinations of powers of these constants.

Planck units: Planck units are a system of units used in particle physics and cosmology. They are defined by four fundamental constants of nature: c, G, ħ, and kB, such that expressing any of these constants in Planck units yields a value of 1. Proposed in 1899 by Max Planck, they provide a natural way to measure physical quantities based on universal properties, rather than on specific objects. Planck units are particularly useful in studying unified theories like quantum gravity.

Degree (angle): A degree (°) is a unit used to measure angles, where a complete rotation is 360 degrees.

Radian: The radian is the standard unit of angle in mathematics. It is defined as the angle subtended at the center of a circle by an arc equal to the radius. It is a dimensionless unit expressed in terms of meters per meter. Radians are commonly used in mathematical writing to measure angles without explicitly specified units.

Steradian: The steradian is the SI unit for measuring solid angles in three-dimensional geometry. It is equivalent to the radian for planar angles. While radians measure circular arc length on a circle, steradians measure the area of a spherical cap on a sphere. The term comes from Greek, combining "stereos" meaning "solid" and "radian."

Metre: The metre is the fundamental unit of length in the SI system. It is defined as the distance traveled by light in vacuum during a specific time interval. This time interval corresponds to 1/299792458 of a second, which is determined by the frequency of a specific caesium transition.

Inch: The inch is a unit of length in the British imperial and US customary systems, equal to 1/36 yard or 1/12 foot. It is derived from the Roman uncia, meaning "twelfth", and is occasionally used to translate similar units in other systems, representing the width of a human thumb.

Foot (unit): The foot (ft) is a unit of length in the British imperial and United States customary systems. It is commonly represented by the prime symbol (′) and is equal to 12 inches. One yard consists of three feet. Since 1959, the foot has been defined as exactly 0.3048 meters.

Cubit: The cubit is an ancient unit of length based on the distance from elbow to fingertip. It was used by Sumerians, Egyptians, and Israelites, and is mentioned in the Bible for Noah's Ark, Ark of the Covenant, Tabernacle, and Solomon's Temple. The common cubit was 24 digits, while royal cubits added a palm for 28 digits. Cubits ranged from 44.4 to 52.92 cm, with ancient Roman cubits as long as 120 cm.

Mile: The mile is a British and American unit of distance based on the older English measurement of 5,280 feet or 1,760 yards. It is also known as the international mile or statute mile. In 1959, it was internationally standardized to be exactly 1,609.344 meters in relation to SI units.

Nautical mile: A nautical mile is a unit of length used in navigation. It was historically defined as the meridian arc length corresponding to one minute of latitude at the equator, making Earth's polar circumference roughly 21,600 nautical miles. The international nautical mile is now defined as 1,852 meters. Speed is measured in knots, which is equivalent to one nautical mile per hour.

Astronomical unit: The astronomical unit is a unit of distance, approximately equal to the average distance from Earth to the Sun, which is about 150 billion meters or 8.3 light-minutes. It represents the varying distance as Earth orbits the Sun throughout the year. In 2012, the astronomical unit was defined precisely as 149,597,870,700 meters.

Light-year: A light-year (ly) is a unit of length used to measure astronomical distances, equal to about 9.5 trillion kilometers or 5.88 trillion miles. It represents the distance light travels in one Julian year, but is often mistakenly thought of as a unit of time.

Parsec: A parsec is a unit of length used in astronomy to measure distances to objects outside our Solar System. It is equal to approximately 3.26 light-years or 30.9 trillion kilometers. The parsec is obtained through parallax and trigonometry, based on the angle at which 1 astronomical unit (AU) subtends. Proxima Centauri, the closest star to the Sun, is about 1.3 parsecs away. Most naked-eye visible stars are within a few hundred parsecs of the Sun, while the farthest known object, the Andromeda Galaxy, is over 700 thousand parsecs away.

Acre: The acre is a unit of land area used in the British and US customary systems. It equals 10 square chains, 1/640 of a square mile, 4,840 square yards, or 43,560 square feet. It's approximately 4,047 m2 or 40% of a hectare. The abbreviation ac is sometimes used, but it is usually spelled out as "acre."

Hectare: A hectare is a non-SI metric unit of area equal to 10,000 square meters, primarily used to measure land. It is equivalent to a square with 100-meter sides. One square kilometer contains 100 hectares. An acre is approximately 0.405 hectares, and a hectare contains around 2.47 acres.

Litre: The litre (L) is a metric unit of volume. It is equal to 1 cubic decimetre (dm3), 1000 cubic centimetres (cm3), or 0.001 cubic metre (m3). It is derived from a cubic decimetre, which is a volume of 10 cm × 10 cm × 10 cm and is one-thousandth of a cubic metre.

Gallon: The gallon is a unit of volume used in British imperial and US customary systems. It has three versions: the imperial gallon (4.54609 litres) used in the UK, Ireland, Canada, Australia, New Zealand, and some Caribbean countries; the US gallon (231 cubic inches) used in the US, Latin American, and Caribbean countries; and the US dry gallon (1/8 US bushel).

Watt: The watt is the SI unit for power, equal to 1 joule per second. It measures the rate of energy transfer and is named after James Watt, a Scottish inventor who played a crucial role in the Industrial Revolution by improving steam engines.

Joule: The joule is the unit of energy in the SI system and is equal to the work done when a force of one newton moves an object by one meter. It is also the energy produced as heat when one ampere of current flows through one ohm of resistance for one second. Named after James Prescott Joule.

Calorie: The calorie is a unit of energy derived from the caloric theory of heat. It measures the amount of heat required to raise the temperature of water by a certain degree. The large calorie is used in food and diet contexts, equivalent to 1000 small calories.

Electronvolt: An electronvolt (eV) is a unit of energy used to measure the amount of kinetic energy gained by a single electron. It is equivalent to the energy gained when an electron passes through an electric potential difference of one volt in vacuum. 1 eV is equal to the charge of an electron in coulombs. According to the 2019 redefinition of SI base units, 1 eV is precisely equal to 1.602176634×10−19 J.

Pascal (unit): The pascal is the SI unit of pressure and is named after Blaise Pascal. It is equal to one newton per square meter (N/m2). It is used to measure internal pressure, stress, Young's modulus, and ultimate tensile strength. It is also equivalent to 10 barye in the CGS system. Common multiples of the pascal include the hectopascal and kilopascal.

Standard atmosphere (unit): The standard atmosphere is a pressure unit equal to 101325 Pa. It serves as a reference for pressure and approximates the average atmospheric pressure at sea level.

Newton (unit): The newton is the SI unit of force, defined as the force required to give a 1 kg mass an acceleration of 1 m/s². It is named after Isaac Newton, who formulated the second law of motion in classical mechanics.

Dalton (unit): The dalton (unit) is a non-SI unit of mass. It is defined as 1/12 of the mass of an unbound neutral atom of carbon-12 in its ground state and at rest. It is also referred to as the unified atomic mass unit. The atomic mass constant, denoted mu, is equivalent to 1 dalton.

Kilogram: The kilogram, symbol kg, is the fundamental unit of mass in the SI system. Used globally in various fields, it is commonly referred to as a kilo. It corresponds to one thousand grams.

Ton: A ton is a unit of measurement for mass, volume, or force with various meanings and uses.

Ounce: The ounce is a measurement unit used for mass, weight, or volume, originating from the Ancient Roman uncia. It has multiple variations and remains largely unaltered throughout history.

Pound (mass): The pound or pound-mass is a unit of mass used in both the British imperial and United States customary systems of measurement. It is defined as exactly 0.45359237 kilograms, and is divided into 16 avoirdupois ounces. The symbol for the avoirdupois pound is lb, but alternative symbols are lbm, #, and ℔ or ″̶.

Knot (unit): The knot is a unit of speed equal to one nautical mile per hour. It is commonly represented by the symbol kn or kt. The knot is used in meteorology, maritime, and air navigation. It is non-SI and widely accepted in aviation.

Decibel: A decibel (dB) is a unit of measurement that represents the ratio of two power or root-power values on a logarithmic scale. It is equal to one-tenth of a bel (B). A difference of one decibel indicates a power ratio of 101/10 or a root-power ratio of 101⁄20.

Second: The second is the SI unit of time, representing 1⁄86400 of a day. It originates from dividing a day into 24 hours, 60 minutes, and 60 seconds each.

Minute: The minute is a unit of time equal to 60 seconds. It is accepted for use in the International System of Units (SI) and is symbolized as min. In the UTC time standard, a minute can sometimes have 61 seconds due to leap seconds, and there is a provision for a negative leap second resulting in a 59-second minute, though this has never occurred in more than 40 years.

Hour: An hour is a unit of time equivalent to 1/24 of a day or 3,600 seconds. It consists of 60 minutes and there are 24 hours in a day.

Day: A day is the 24-hour period corresponding to a complete rotation of the Earth around the Sun. It includes morning, noon, afternoon, evening, and night, shaping the circadian rhythms in various organisms and impacting essential life processes.

Week: A week is a period of seven days commonly used for work, rest, and worship around the world. Weeks are not based on astronomy but are often aligned with yearly calendars.

Month: A month is a unit of time used in calendars, roughly equivalent to the natural orbital period of the Moon. The concept of months originated from the cycle of Moon phases, known as synodic months, which last around 29.53 days. This led people in the Paleolithic age to count days based on lunar phases. Nowadays, synodic months remain a fundamental basis for many calendars, dividing the year into approximately 12.37 months.

Year: A year represents the duration needed for celestial objects to complete an orbit. On Earth, it corresponds to the time taken for Earth to revolve around the Sun. Besides referring to a calendar year, the term encompasses various periods related to the calendar or astronomy, like seasonal, fiscal, and academic years. Additionally, it can denote any lengthy cycle, such as the Great Year.

Hertz: The hertz is the unit of frequency in the International System of Units (SI), equal to one event per second. It is named after Heinrich Rudolf Hertz, who proved the existence of electromagnetic waves. Hertz are commonly expressed in multiples: kilohertz (kHz), megahertz (MHz), gigahertz (GHz), terahertz (THz).

Coulomb: The coulomb (C) is the SI unit for electric charge. It is the charge delivered by 1 ampere current within 1 second and is defined in relation to the elementary charge e. Its value is approximately 6.241509×10^18 e.

Ampere: The ampere (amp) is the SI unit of electric current. It is equal to 1 coulomb moving past a point in 1 second. Named after André-Marie Ampère, a French mathematician and physicist, who is a renowned contributor to electromagnetism.

Volt: The volt is the SI unit for electric potential, voltage, and electromotive force.

Farad: The farad (F) is the unit of electrical capacitance, measuring an object's ability to store an electrical charge. It is equivalent to 1 coulomb per volt (C/V) and is named after the physicist Michael Faraday. In SI base units, 1 F = 1 kg−1⋅m−2⋅s4⋅A2.

Tesla (unit): Tesla is the SI unit for measuring magnetic flux density.

Weber (unit): The weber unit is used to measure magnetic flux in the SI system, with 1 weber equal to 1 volt-second. It corresponds to a magnetic flux density of 1 tesla.

Ohm: The ohm is the SI unit for electrical resistance, named after physicist Georg Ohm. Standard units for resistance were developed during early telegraphy practice. In 1861, the British Association for the Advancement of Science proposed a practical unit derived from existing units of mass, length, and time.

Siemens (unit): The siemens, symbolized as S, is the unit of electric conductance, susceptance, and admittance in the International System of Units (SI). It is equal to the reciprocal of one ohm and is also known as the mho. The siemens was adopted by the IEC in 1935 and approved as a derived unit in 1971 by the 14th General Conference on Weights and Measures.

Henry (unit): The henry is the SI unit of electrical inductance. It measures the self inductance of a coil when a current of 1 ampere produces 1 weber turn of flux linkage. The unit is named after Joseph Henry, an American scientist who independently discovered electromagnetic induction around the same time as Michael Faraday in England.

Celsius: The Celsius scale is a temperature scale used worldwide, alongside the Kelvin scale. It is named after astronomer Anders Celsius and is commonly referred to as Centigrade. This scale measures specific temperatures and temperature differences between two points.

Fahrenheit: The Fahrenheit scale is a temperature scale created by physicist Daniel Gabriel Fahrenheit in 1724. It uses the degree Fahrenheit as its unit. The scale's lower defining point, 0 °F, represents the freezing temperature of a brine solution. The upper limit was initially set at 90 °F, later adjusted to 96 °F, to approximate average human body temperature.

Kelvin: The kelvin, symbol K, is a unit of temperature used in the International System of Units (SI). It is an absolute scale, starting at absolute zero. A change of 1 kelvin corresponds to a change of thermal energy by 1.380649×10−23 J. The kelvin is named after William Thomson, also known as Lord Kelvin, a notable engineer and physicist.

Mole (unit): The mole is the unit of measurement for the amount of substance in the International System of Units (SI). One mole contains approximately 602 sextillion elementary entities, such as atoms or molecules. The Avogadro number represents the number of particles in a mole. Originally based on 12 grams of carbon-12, the mole's value is now approximate due to recent redefinitions of SI base units.

Candela: The candela is the unit of luminous intensity in the SI system. It measures the power of light emitted by a source in a specific direction. Luminous intensity considers the sensitivity of the human eye to different wavelengths of light. The CIE and ISO have standardized this sensitivity. A typical wax candle emits light with a luminous intensity of about one candela. Even if some directions are blocked by an obstacle, the emission in unobscured directions remains approximately one candela.

Lumen (unit): The lumen is the unit of luminous flux, measuring the total visible light emitted by a source per unit of time. It differs from power as it accounts for the human eye's sensitivity to various wavelengths. This unit is standardized by the CIE and ISO. One lux is equal to one lumen per square meter.

Lux: The lux is the SI unit of illuminance, measuring the intensity of light on a surface. It is equal to one lumen per square metre. Lux is used to gauge human visual brightness perception, with its value weighted according to the CIE and ISO luminosity function. Its name is derived from the Latin word for "light".

Becquerel: The becquerel is the SI unit of radioactivity. It measures the number of radioactive decays per second. It is commonly used for human health applications.

Gray (unit): The gray (unit) is the SI unit for measuring ionizing radiation dose. It signifies the absorption of one joule of radiation energy per kilogram of matter.

Sievert: The sievert is a unit in the SI that measures the health risks of ionizing radiation. It represents the likelihood of causing radiation-induced cancer and genetic damage. The sievert is crucial for dosimetry and radiation protection. It is named after Rolf Maximilian Sievert, a prominent Swedish medical physicist who studied radiation dose measurement and the biological effects of radiation.

Bit: A bit is the fundamental unit of information in computing and digital communications, derived from binary digits. It represents a logical state with two possible values, typically "1" or "0", but can also be represented by true/false, yes/no, on/off, or +/−.

Byte: A byte is a unit of digital information that typically consists of eight bits. It was historically used to encode a single character of text and is the smallest addressable unit of memory in many computer systems. To avoid confusion with other byte sizes, an 8-bit byte is often referred to as an octet in network protocols. The bits in an octet are counted with numbering from 0 to 7 or 7 to 0, depending on the bit endianness.

Astronomy: Astronomy is a natural science that explores celestial objects and phenomena in the cosmos. It uses mathematics, physics, and chemistry to study the origin and evolution of planets, stars, galaxies, and more. This includes supernovas, gamma ray bursts, quasars, and cosmic microwave background radiation. Cosmology, a subset of astronomy, studies the entire universe.

Astronomical object: An astronomical object is a naturally occurring structure in the observable universe, while an astronomical body is a single, tightly bound entity. Objects can be complex and consist of multiple bodies or other substructures.

Astrophysics: Astrophysics is a scientific field that uses principles from physics and chemistry to study celestial objects and phenomena. It aims to understand the nature of these heavenly bodies rather than just their positions in space. Astrophysicists investigate various subjects including the Sun, stars, galaxies, planets outside our solar system, interstellar medium, and cosmic microwave background. They examine the emissions of these objects and analyze properties such as luminosity, density, temperature, and chemical composition across the electromagnetic spectrum. This interdisciplinary field incorporates concepts and methods from classical mechanics, electromagnetism, statistical mechanics, thermodynamics, quantum mechanics, relativity, nuclear and particle physics, and atomic and molecular physics.

Extraterrestrial life: Extraterrestrial life refers to life not from Earth. It has not been proven to exist yet, but may range from basic organisms to advanced civilizations. The Drake equation speculates on the possibility of intelligent life elsewhere. This field of study is called astrobiology.

Observatory: An observatory is a place dedicated to observing terrestrial, marine, or celestial events. It is used for various scientific disciplines such as astronomy, climatology, geophysics, oceanography, and volcanology. Observatories have evolved from simple instruments like the sextant to sophisticated facilities.

Luminosity: Luminosity is the measure of electromagnetic energy emitted by an object, such as a star or galaxy. It represents the total amount of radiant power per unit time and is commonly used in astronomy to describe the brightness of celestial bodies.

Magnitude (astronomy): Magnitude in astronomy is a measure of an object's brightness in a specific passband. It was first introduced by Hipparchus in ancient times.

Extinction (astronomy): Extinction in astronomy refers to the absorption and scattering of electromagnetic radiation by dust and gas between an object and the observer. Its effects on star colors were observed prior to 1930, but it was officially documented by Robert Julius Trumpler. For stars near the Milky Way's plane within a few thousand parsecs of Earth, the visual extinction is about 1.8 magnitudes per kiloparsec.

Photometry (astronomy): Photometry is an astronomy technique for measuring the intensity of light emitted by celestial objects. It relies on a photometer, which converts light into an electric current, often using electronic devices like CCD or photoelectric photometers. By calibrating against standard stars, photometers can determine the brightness or apparent magnitude of astronomical objects.

Cosmic distance ladder: The cosmic distance ladder is a set of methods used by astronomers to measure distances to celestial objects. Close objects can be directly measured, while distant objects are determined based on correlations between close and far distance methods. Standard candles, known luminous objects, are often used to estimate distances accurately.

Astrometry: Astrometry is a branch of astronomy focused on accurately measuring positions and movements of celestial bodies. It offers valuable insights into the kinematics and origins of our Solar System and the Milky Way galaxy.

Parallax: Parallax is a displacement in the apparent position of an object seen from two different viewpoints. It is measured by the angle or half-angle of inclination between these viewpoints. Parallax can be used to determine distances, as nearby objects exhibit more noticeable parallax than farther ones.

Solar System: The Solar System is composed of the Sun and its orbiting objects, including eight planets categorized into four terrestrial planets, two gas giants, and two ice giants. It originated 4.6 billion years ago from the collapse of a dense molecular cloud, resulting in the formation of the Sun and a protoplanetary disc.

Formation and evolution of the Solar System: The Solar System formed 4.6 billion years ago from a collapsing part of a giant molecular cloud. The Sun formed in the center while the remaining material flattened into a disk, which later gave rise to planets, moons, asteroids, and other objects in the Solar System.

Nebular hypothesis: The nebular hypothesis is a widely accepted model explaining the formation and evolution of the Solar System. It proposes that the planets formed from gas and dust orbiting the Sun, clumping together over time. Originally developed by Immanuel Kant and modified by Pierre Laplace, it has been expanded to apply to planetary system formation throughout the universe. The modern variant, known as the solar nebular disk model, explains various properties of the Solar System, such as the planets' nearly circular and coplanar orbits, and their motion in the same direction as the Sun's rotation. While some aspects of the original theory remain relevant, many have been replaced by newer ideas in planetary formation.

Interplanetary medium: The interplanetary medium is the substance that fills the Solar System and allows planetary bodies to move. It extends until the heliopause, beyond which the interstellar medium starts. It was previously believed to either be a vacuum or composed of "aether."

Sun: The Sun is a massive hot star at the center of the Solar System. It produces energy through fusion reactions at its core and emits light, ultraviolet, and infrared radiation. This radiation provides the majority of Earth's energy for life. The Sun functions as a magneto-alternator.

Solar wind: The solar wind is a stream of charged particles released from the Sun's corona, consisting of electrons, protons, and alpha particles. It also contains trace amounts of heavy ions and atomic nuclei from elements like C, N, O, Ne, Mg, Si, S, and Fe. The solar wind's density, temperature, and speed vary over time and space due to the interplanetary magnetic field. These particles can escape the Sun's gravity because of their high energy from the corona's temperature, which is influenced by the coronal magnetic field. The boundary between the corona and the solar wind is known as the Alfvén surface.

Mercury (planet): Mercury is the smallest planet closest to the Sun. It has a heavily cratered surface with no geological activity. Despite its small size, it has a similar surface gravity to Mars. Mercury has a thin atmosphere called an exosphere and a weak magnetic field. It also lacks any natural satellites.

Venus: Venus, the second planet from the Sun, is known for its dense atmosphere and volcanic surface. With a diameter similar to Earth's, this terrestrial planet boasts 167 volcanoes over 100 km wide. Its atmosphere is so dense that it becomes a supercritical fluid at surface level and 92 atmospheres.

Earth: Earth is the third planet from the Sun and the only one known to have life. It is a water world, with a global ocean covering 70.8% of its surface. The remaining 29.2% is land, mostly in the form of continental landmasses. Earth's land is humid and covered in vegetation, while polar ice sheets hold more water than all other sources combined. Tectonic plates create mountains, volcanoes, and earthquakes. Earth has a liquid outer core that produces a protective magnetosphere against solar winds and cosmic radiation.

Moon: The Moon is Earth's only natural satellite, orbiting at an average distance of 384,400 km. Its rotation is locked to Earth, resulting in a lunar day matching the lunar month of 29.5 Earth days. The Moon's gravitational pull, along with the Sun's, drives the tides.

Mars: Mars, the fourth planet from the Sun, is known as the Red Planet due to its orange-red surface covered in iron(III) oxide dust. It boasts massive extinct volcanoes and one of the largest canyons in the Solar System. With a diameter of 6,779 km (4,212 mi), Mars is the second smallest terrestrial planet in our Solar System.

Asteroid belt: The asteroid belt is a region between the orbits of Jupiter and Mars that is filled with numerous irregularly-shaped objects called asteroids or minor planets. It is a torus-shaped area centered on the Sun, containing objects much smaller than planets and situated about one million kilometers apart on average. Also known as the main asteroid belt, it differentiates from other asteroid groups in the Solar System.

Ceres (dwarf planet): Ceres is a dwarf planet located in the main asteroid belt between Mars and Jupiter. It was first discovered in 1801 and initially considered a new planet. Later, it was reclassified as an asteroid and then as a dwarf planet, unique for always being inside Neptune's orbit.

Jupiter: Jupiter is the largest planet in our Solar System and orbits the Sun at a distance of 5.20 AU. It has a mass greater than all the planets combined and is about one one-thousandth the mass of the Sun. Jupiter is the third brightest object in the night sky and has been observed since ancient times. It was named after the chief Roman god, Jupiter.

Io (moon): Io is the innermost and third-largest moon of Jupiter, known for its high density, strong surface gravity, and scarcity of water. It is larger than Earth's moon and ranks fourth in size among all moons in the Solar System. Discovered by Galileo Galilei in 1610 and named after the mythological character Io, it was a priestess of Hera and became a lover of Zeus.

Europa (moon): Europa is a moon of Jupiter, the smallest of the four Galilean moons. It is the sixth-closest moon to Jupiter and the sixth-largest moon in our Solar System. Europa was discovered by both Simon Marius and Galileo Galilei and is named after a Phoenician figure from Greek mythology.

Ganymede (moon): Ganymede is Jupiter's largest moon and the biggest in the entire Solar System. It lacks an atmosphere, yet possesses a notable magnetic field. With its lower density, it is larger than Mercury but has lesser surface gravity than Mercury, Io, and the Moon.

Callisto (moon): Callisto is Jupiter's second-largest moon, and the third-largest moon in the Solar System. It is roughly a third larger than Earth's Moon and orbits Jupiter at a distance of 1883000 km. Callisto is part of the four large Galilean moons, discovered in 1610, and is visible from Earth with binoculars.

Saturn: Saturn, the second-largest planet in our Solar System, is a gas giant located sixth from the Sun. With an average radius almost 9.5 times that of Earth, it's significantly less dense but over 95 times more massive.

Enceladus: Enceladus is Saturn's sixth-largest moon, with a diameter of 500 kilometers. Covered mostly by clean ice, it is highly reflective. Its surface temperature reaches a frigid -198 °C during noon. Enceladus showcases diverse surface features, including both old, cratered regions and younger, tectonically deformed terrain.

Titan (moon): Titan (moon) is Saturn's largest moon and the Solar System's second-largest. It boasts a dense atmosphere and is the only known celestial object, aside from Earth, with stable surface liquid bodies.

Uranus: Uranus is the seventh planet from the Sun, known for its gaseous composition and cyan color. It is primarily made up of water, ammonia, and methane in an ice-like state. The planet has a complex cloud structure and the coldest minimum temperature among all Solar System planets. With its unique axial tilt of 82.23° and retrograde rotation rate of 17 hours, Uranus experiences extreme seasons where its poles go through 42 years of continuous sunlight followed by 42 years of darkness during its 84-year orbit around the Sun.

Neptune: Neptune is the eighth and farthest planet from the Sun. It is the fourth-largest and third-most-massive planet in the Solar System. Being denser and smaller than its near-twin Uranus, Neptune has no well-defined solid surface. It orbits the Sun every 164.8 years at a distance of 30.1 astronomical units. Named after the Roman god of the sea, Neptune's symbol is a trident.

Triton (moon): Triton is Neptune's largest moon, discovered in 1846 by William Lassell. It stands out for having a unique retrograde orbit opposite to its planet's rotation. With a composition similar to Pluto, it is believed to have been captured from the Kuiper belt, potentially making it a dwarf planet.

Kuiper belt: The Kuiper belt is a large disc in the outer Solar System, located between Neptune and the Sun. It is much wider and more massive than the asteroid belt. Composed of small bodies, it formed during the early stages of the Solar System. Unlike asteroids, Kuiper belt objects are mostly made of frozen volatiles. This region is home to dwarf planets like Pluto, Orcus, Haumea, Quaoar, and Makemake. Additionally, some of the Solar System's moons, including Triton and Phoebe, may have originated from the Kuiper belt.

Pluto: Pluto is a dwarf planet located in the Kuiper belt, beyond Neptune's orbit. It is the ninth-largest and tenth-most-massive object directly orbiting the Sun. Despite being slightly smaller and less massive than Eris, it is the largest known trans-Neptunian object in terms of volume. Similar to other objects in the Kuiper belt, Pluto is primarily composed of ice and rock. It is significantly smaller than the inner planets and only has one sixth the mass of Earth's moon, with one third of its volume.

Eris (dwarf planet): Eris is a dwarf planet in the Solar System, known for being the most massive after Pluto. Its high-eccentricity orbit places it in the scattered disk, and it was discovered in 2005. Eris, named after the goddess of strife, is the 9th most massive object orbiting the Sun and the largest unvisited by a spacecraft. It measures 2,326 km in diameter, making it slightly smaller than Pluto but with a greater mass. Both Eris and Pluto have surface areas comparable to Russia or South America.

Oort cloud: The Oort cloud, also known as the Öpik–Oort cloud, is a vast icy cloud surrounding the Sun. It extends from 2,000 to 200,000 astronomical units (AU) away. Proposed by Jan Oort in 1950, this cloud is thought to contain planetesimals that replenish the number of long-period comets entering the inner Solar System. These comets are eventually destroyed during their close encounters with the Sun.

Halley's Comet: Halley's Comet, officially known as 1P/Halley, is a short-period comet that appears every 75-79 years. It is the only known comet visible to the naked eye and can be seen twice in a human lifetime. Last seen in 1986, it is expected to reappear in mid-2061.

Alpha Centauri: Alpha Centauri is a triple star system in the constellation of Centaurus. It includes three stars: Rigil Kentaurus, Toliman (B), and Proxima Centauri (C). Proxima Centauri holds the distinction of being the closest star to the Sun, located 4.2465 light-years away.

Arcturus: Arcturus is the brightest star in the constellation of Boötes, with a visual magnitude of −0.05. It is the fourth-brightest star in the night sky and the brightest in the northern celestial hemisphere. Originating from ancient Greece, it is also known as α Boötis or Alpha Boötis. Arcturus is a corner of the Spring Triangle asterism.

Betelgeuse: Betelgeuse is a red supergiant star in the Orion constellation. It is one of the largest stars visible to the naked eye and is ten times more massive than the Sun. It is the tenth-brightest star in the night sky, appearing distinctly reddish. Betelgeuse varies in brightness, with the widest range displayed by any first-magnitude star. It is the brightest star in the night sky at near-infrared wavelengths. Its Bayer designation is α Orionis, also known as Alpha Orionis or α Ori.

Canopus: Canopus, also known as α Carinae, is the brightest star in the southern constellation of Carina and the second-brightest star in the night sky with a visual magnitude of −0.74. It holds its prominence only to Sirius.

Capella: Capella is the brightest star in the constellation Auriga. It is the sixth-brightest star in the night sky and the third-brightest in the northern celestial hemisphere. Capella is circumpolar to observers north of 44°N and is a notable object in the winter sky. Its name means "little goat" and it represents the goat that nursed Zeus in ancient mythology. Capella is located relatively close to the Sun, about 42.9 light-years away. It is also a significant source of X-rays, primarily emitting from its corona.

Cygnus X-1: Cygnus X-1 is a galactic X-ray source in the constellation Cygnus that is widely accepted to be a black hole. It was discovered in 1971 and remains one of the strongest X-ray sources detectable from Earth. With a mass about 21.2 times that of the Sun, it is too small to be any known kind of normal star or object. The source produces occasional X-ray bursts lasting only for about 1 ms, pointing towards a compact object with an event horizon radius of 300 km. Overall, Cygnus X-1 is a highly studied astronomical object, contributing to our understanding of black holes.

Polaris: Polaris is a bright star in Ursa Minor and is commonly known as the North Star or Pole Star. It is the brightest star in its constellation and can be seen easily with the naked eye. Polaris is located very close to the north celestial pole, making it the current northern pole star and useful for navigation.

Procyon: Procyon is the brightest star in the constellation Canis Minor, and one of the closest stars to Earth at a distance of 11.46 light-years. It is commonly known as α Canis Minoris or Alpha CMi.

Rigel: Rigel is a blue supergiant star known as β Orionis or Beta Orionis. It is the brightest and most massive component of a star system in the constellation of Orion. This system, located 860 light-years away, appears as a single blue-white point of light to the naked eye.

Sirius: Sirius is the brightest star in the night sky, named after the Greek word for "glowing" or "scorching." It is designated as α Canis Majoris and has an apparent magnitude almost twice as bright as Canopus. This binary star consists of a main-sequence star, Sirius A, and a faint white dwarf companion, Sirius B, which orbit each other every 50 years.

Vega: Vega is the brightest star in the Lyra constellation, designated as α Lyrae. It is located 25 light-years from the Sun, making it relatively close. Vega is one of the most luminous stars nearby and ranks as the fifth-brightest star at night, and the second-brightest in the northern celestial hemisphere next to Arcturus.

Milky Way: The Milky Way is our galaxy, visible as a hazy band of light in the night sky. Its name is a translation of Latin and Greek terms meaning "milky circle." Galileo first saw individual stars within this band in 1610. Before the 1920s, people believed that the Milky Way contained all the stars in the Universe, but Edwin Hubble proved that it is just one of many galaxies.

Galactic Center: The Galactic Center is the central rotational point and barycenter of the Milky Way. It contains a supermassive black hole called Sagittarius A*, weighing 4 million times the mass of our sun. Located about 26,000 light-years away in the direction of constellations Sagittarius, Ophiuchus, and Scorpius, it is visually brightest near the Butterfly Cluster (M6) or the star Shaula, south to the Pipe Nebula.

Sagittarius A*: Sagittarius A* (Sgr A*) is the supermassive black hole at the center of our Milky Way galaxy. Situated near the border of Sagittarius and Scorpius constellations, it lies about 5.6° south of the ecliptic and is visually close to the Butterfly Cluster (M6) and Lambda Scorpii.

Local Group: The Local Group is a galaxy group including the Milky Way and Andromeda Galaxy with a total diameter of 3 megaparsecs and a mass of 2×10^12 solar masses. It consists of two collections in a "dumbbell" shape, separated by 800 kiloparsecs and moving towards each other at a speed of 123 km/s. It is part of the larger Virgo Supercluster and is home to at least 80 known galaxies, mostly dwarf galaxies.

Andromeda Galaxy: The Andromeda Galaxy is the nearest major galaxy to the Milky Way. Originally called the Andromeda Nebula, it is a barred spiral galaxy known as Messier 31, M31, and NGC 224. With a diameter of about 46.56 kiloparsecs, it lies approximately 765 kpc from Earth. Its name comes from the constellation of Andromeda, where it appears in the sky. The galaxy is named after a princess in Greek mythology who was married to Perseus.

Large Magellanic Cloud: The Large Magellanic Cloud (LMC) is a satellite galaxy of the Milky Way, located about 163,000 light-years away. It is the second- or third-closest galaxy to the Milky Way. The LMC is approximately 32,200 light-years across and is one-hundredth the mass of the Milky Way. It ranks as the fourth-largest galaxy in the Local Group, after the Andromeda Galaxy, the Milky Way, and the Triangulum Galaxy.

Small Magellanic Cloud: The Small Magellanic Cloud (SMC) is a dwarf galaxy located near the Milky Way. It is classified as a dwarf irregular galaxy and contains several hundred million stars. With a diameter of about 5.78 kiloparsecs (18,900 light-years) and a mass of approximately 7 billion solar masses, it is among the nearest intergalactic neighbors of the Milky Way. Despite being about 200,000 light-years away, it is one of the most distant objects visible to the naked eye.

Triangulum Galaxy: The Triangulum Galaxy, also known as Messier 33 or NGC 598, is a spiral galaxy located 2.73 million light-years away in the constellation Triangulum. It is the third-largest member of the Local Group of galaxies, after the Andromeda Galaxy and the Milky Way. It has an isophotal diameter of 18.74 kiloparsecs (61,100 light-years).

3C 273: 3C 273 is a quasar in the Virgo constellation. It is the first quasar discovered and the brightest one from Earth, with a visual magnitude of 12.9. It is located at the center of a giant elliptical galaxy. Its estimated distance is 749 megaparsecs and it has a central supermassive black hole with a mass of approximately 886 million times that of the Sun.

Centaurus A: Centaurus A is a galaxy in the Centaurus constellation, discovered in 1826 by James Dunlop. Its fundamental properties are still debated, but it is known to have a close proximity to Earth and has been extensively studied by astronomers. It is the fifth-brightest galaxy in the sky and is a favored target for amateur astronomers in the southern hemisphere and low northern latitudes.

Messier 87: Messier 87 is an enormous elliptical galaxy in the Virgo constellation, housing trillions of stars. As one of the largest and most massive galaxies in our vicinity, it boasts an abundance of globular clusters, with around 15,000 clusters compared to the 150-200 surrounding our Milky Way galaxy. This galaxy also showcases a high-energy plasma jet originating from its core, stretching over 1,500 parsecs and traveling at a relativistic speed. Additionally, Messier 87 is among the most prominent radio sources in the sky, captivating the attention of both professional and amateur astronomers.

Pinwheel Galaxy: The Pinwheel Galaxy is a spiral galaxy located 21 million light-years away in Ursa Major. Discovered by Pierre Méchain in 1781, it was later included in the Messier Catalogue.

Whirlpool Galaxy: The Whirlpool Galaxy, or Messier 51a (M51a) in the constellation Canes Venatici, is a grand-design spiral galaxy. It is known for being the first galaxy classified as a spiral and contains a Seyfert 2 active galactic nucleus. Located 32 million light-years away, it spans a diameter of 109,000 ly.

Carina Nebula: The Carina Nebula, found in the Carina constellation, is a massive and intricate region filled with bright and dark nebulosity. Situated in the Carina-Sagittarius arm of the Milky Way galaxy, it resides about 8,500 light-years away from Earth.

Crab Nebula: The Crab Nebula is a supernova remnant in the constellation Taurus. It was discovered in 1731 and corresponds with a bright supernova observed by Chinese astronomers in 1054. Its common name comes from a drawing resembling a crab made in 1842. It was the first astronomical object linked to a historically-recorded supernova explosion.

Eagle Nebula: The Eagle Nebula is a young cluster of stars in Serpens. It contains "Pillars of Creation," known for their dark silhouette, as imaged by the Hubble Telescope. This nebula has active regions of gas and dust involved in star formation. It is located in the Sagittarius Arm of the Milky Way.

Horsehead Nebula: The Horsehead Nebula is a dark nebula in the constellation Orion, situated south of Alnitak, the eastern star in Orion's Belt. It belongs to the vast Orion molecular cloud complex. Found within the dust cloud Lynds 1630, it borders the active star-forming H II region known as IC 434.

Orion Nebula: The Orion Nebula is a bright and easily observable diffuse nebula located in the Milky Way, in the constellation of Orion. It is the middle "star" in Orion's "sword," visible to the naked eye with an apparent magnitude of 4.0. Situated about 1,344 light-years away, it is the closest area of massive star formation to Earth. The M42 nebula spans approximately 24 light-years and has a mass 2,000 times that of the Sun. It is often referred to as the Great Nebula in Orion.

Ring Nebula: The Ring Nebula in Lyra is created by a dying star expelling a luminous gas envelope before becoming a white dwarf.

Hyades (star cluster): The Hyades is a well-studied star cluster that is the closest of its kind to Earth, located approximately 153 light-years away. It is a spherical group of hundreds of stars with the same age, origin, chemical properties, and movement in space. In the constellation Taurus, the Hyades Cluster forms a "V" shape alongside the brighter star Aldebaran, although they are not directly related as Aldebaran is much closer to Earth by coincidence.

Omega Centauri: Omega Centauri is a massive globular cluster in the Centaurus constellation. It was first recognized as a non-stellar object by Edmond Halley in 1677. The cluster is located 17,090 light-years away and has a diameter of approximately 150 light-years. With around 10 million stars and a total mass equivalent to 4 million solar masses, it holds the record for being the largest and most massive globular cluster in our galaxy, the Milky Way.

Pleiades: The Pleiades, also known as the Seven Sisters, is an open star cluster located in the Taurus constellation. It is one of the nearest star clusters to Earth, lying about 444 light years away. The Pleiades is easily visible to the naked eye and is the closest Messier object. Additionally, it contains hot B-type stars and is home to the reflection nebula NGC 1432.

Virgo Supercluster: The Virgo Supercluster, also known as the Local Supercluster, is a large collection of galaxies that includes the Virgo Cluster and Local Group, which contains the Milky Way and Andromeda galaxies. It encompasses at least 100 galaxy groups and clusters within a diameter of 33 megaparsecs. It is one of about 10 million superclusters in the observable universe and is part of the Pisces-Cetus Supercluster Complex, a galaxy filament.

Virgo Cluster: The Virgo Cluster is a large group of galaxies located about 53.8 million light-years away in the Virgo constellation. It consists of around 1,300 galaxies and serves as the core of the even bigger Virgo Supercluster. The Local Group, including our own Milky Way, is part of the Virgo Supercluster and is affected by its gravitational pull. The mass of the Virgo Cluster is estimated to be about 1.2×1015 times that of our Sun.

Great Attractor: The Great Attractor is a powerful gravitational force in intergalactic space, acting as the central point of the Laniakea Supercluster. It influences the motion of over 100,000 galaxies, including the Milky Way.

Celestial mechanics: Celestial mechanics is a branch of astronomy that studies the motions of objects in outer space. It applies principles of physics to produce ephemeris data for astronomical objects like stars and planets.

Orbit: An orbit is the curved path an object follows around another object in space. This can include planets orbiting a star, satellites orbiting a planet or moon, or artificial satellites orbiting any celestial body. Orbits can be regular or irregular, but most planets and satellites follow elliptical orbits around a focal point determined by the center of mass. Kepler's laws explain their motion.

Barycenter (astronomy): The barycenter in astronomy is the center of mass for multiple orbiting bodies. It is a dynamic point and not a physical object. This concept is important in the fields of astronomy and astrophysics. The distance from a body's center of mass to the barycenter can be calculated using a two-body problem.

Tidal locking: 'Tidal locking' refers to a state where two orbiting celestial bodies stop changing their rotation rate. In synchronous rotation, a tidally locked body takes the same time to rotate around its axis as it does to revolve around its partner. An example is the moon always showing the same face to the Earth. Usually, only the satellite is tidally locked, but under specific conditions, both bodies can be mutually locked. This phenomenon is observed between Pluto and Charon, as well as between Eris and Dysnomia. Other terms for tidal locking include gravitational locking, captured rotation, and spin-orbit locking.

Eclipse: An eclipse is a temporary obscuring of an astronomical object by another body or by the alignment of three celestial objects. This alignment, called syzygy, can result in either an occultation or a transit. A deep eclipse refers to a situation where a smaller astronomical object is hidden by a larger one.

Lunar eclipse: A lunar eclipse is when the Moon enters Earth's shadow, making it appear darkened. It happens every six months during a full moon when the Moon's orbit aligns with Earth's orbit.

Solar eclipse: A solar eclipse is when the Moon passes between the Earth and the Sun, blocking the Sun's view from a small part of the Earth. It happens every six months during the new moon phase and when the Moon's orbit is closest to Earth's orbit. Total eclipses fully obscure the Sun, while partial and annular eclipses only cover part of it. Unlike lunar eclipses visible from anywhere on the night side of Earth, solar eclipses can only be seen from a small area. Total solar eclipses occur every 18 months on average but are only visible from a specific location once every 360 to 410 years.

Kepler's laws of planetary motion: Kepler's laws of planetary motion, formulated by Johannes Kepler in the early 17th century, describe the orbits of planets around the Sun. They replaced circular orbits with elliptical trajectories and explained variations in planetary velocities. The laws state that planets move in ellipses with the Sun at one focus, the line connecting a planet and the Sun sweeps equal areas in equal times, and a planet's orbital period squared is proportional to the cube of the length of its orbit's semi-major axis.

Lagrange point: Lagrange points are equilibrium points in celestial mechanics, where small objects are gravitationally influenced by two larger orbiting bodies. These points are solutions to the restricted three-body problem.

Night: Night is the time from sunset to sunrise when it is dark outside because the Sun is below the horizon. The duration of night depends on the location, season, and latitude.

Orbital mechanics: Orbital mechanics, also known as astrodynamics, applies ballistics and celestial mechanics to solve practical problems related to the motion of spacecraft. Using Newton's laws of motion and the law of universal gravitation, it calculates the trajectory of rockets, satellites, and other objects in space. This discipline is crucial for designing and controlling space missions.

Escape velocity: Escape velocity is the minimum speed required for an unpowered object to break free from the gravitational pull of a central body. It is independent of direction and ignores atmospheric friction. The speed depends on the mass of the central body and decreases with distance traveled. Once an object reaches escape velocity, it does not require further acceleration to escape, but will continuously slow down due to the gravitational force.

Geostationary orbit: A geostationary orbit is a high-altitude circular orbit above Earth's equator that follows the direction of Earth's rotation. It is located 35,786 km in altitude and 42,164 km in radius from Earth's center. It is also known as a geosynchronous equatorial orbit (GEO).

Geosynchronous orbit: A geosynchronous orbit is a type of orbit around the Earth where the object takes the same amount of time as Earth's rotation, about 24 hours, to complete one orbit. This means the object appears to stay in the same position in the sky from Earth's surface. The path of the object can be still or follow a figure-8 shape, depending on the inclination and eccentricity of the orbit. A circular geosynchronous orbit is at a fixed altitude of 35,786 km (22,236 mi).

Gravity assist: Gravity assist is a spaceflight maneuver that utilizes the gravity and movement of planets or other celestial objects to alter a spacecraft's path and speed. This technique helps conserve propellant and reduce expenses.

Low Earth orbit: A Low Earth Orbit (LEO) is a relatively close orbit around Earth with a short period and low ellipticity. Most man-made objects in space are found in LEO, at altitudes up to about one-third of Earth's radius.

Celestial sphere: The celestial sphere is an abstract sphere that encompasses the sky, with all objects appearing as if projected onto its inner surface. It can be centered either on Earth or the observer, resembling a hemispherical screen over the observer's location. This concept is used in astronomy and navigation.

Constellation: A constellation is a group of visible stars that form a pattern on the celestial sphere, often representing animals, mythological subjects, or inanimate objects.

Astronomical coordinate systems: Astronomical coordinate systems are used in astronomy to determine the positions of celestial objects. These systems can specify positions in three-dimensional space or simply the direction on a celestial sphere. They are based on physical reference points available to observers.

Equatorial coordinate system: The equatorial coordinate system is a widely used method to locate celestial objects. It can be defined in spherical or rectangular coordinates, with an origin at Earth's center, a fundamental plane aligned with Earth's equator projected onto the celestial sphere, a primary direction pointing towards the vernal equinox, and a right-handed convention.

Equinox: An equinox is a solar event when the Sun appears directly above the Earth's equator. This happens twice a year, around March 20 and September 23. On these days, the Sun rises and sets exactly in the east and west directions.

Ecliptic: The ecliptic, or orbital plane of Earth around the Sun, is a significant reference plane in astronomy. As the Earth orbits the Sun, the Sun's apparent yearly movement against the backdrop of stars creates the path of the ecliptic. It serves as the foundation for the ecliptic coordinate system, a crucial tool for celestial observations.

Horizon: The horizon is the visible line that appears to separate the sky from the surface of a celestial body when viewed from an observer's perspective. It divides viewing directions based on whether they intersect the body's surface or not.

Solstice: A solstice is when the Sun reaches its farthest point from the celestial equator. It happens twice a year, around June 21 and December 21. Solstices determine seasons in many countries.

Zodiac: The Zodiac is a belt-shaped region in the sky, about 8° north and south of the ecliptic - the Sun's apparent path throughout the year. It contains the orbital paths of the Moon and major planets.

Canis Major: Canis Major is a southern constellation that was included in Ptolemy's 48 constellations. Its name means "greater dog" in Latin. It is often depicted following Orion the hunter in the sky. The Milky Way intersects with Canis Major and it contains various open clusters, including the notable M41 cluster.

Cassiopeia (constellation): Cassiopeia is a constellation in the northern sky named after a vain queen in Greek mythology. It is one of the 88 modern constellations and was listed by the Greek astronomer Ptolemy. Recognizable by its distinctive 'W' shape, formed by five bright stars.

Centaurus: Centaurus is a renowned constellation in the southern sky, ranking among the largest constellations. It has a significant mythological association with the centaur, a half-human, half-horse creature. Noteworthy stars within Centaurus include Alpha Centauri, the closest star system to our Solar System, as well as Beta Centauri and V766 Centauri, one of the largest stars ever observed. Additionally, Omega Centauri, a remarkable globular cluster, shines as the brightest and largest one in our Milky Way, potentially originating from a dwarf galaxy.

Crux: Crux is a small constellation in the southern sky known as the Southern Cross. It contains four bright stars forming a cross shape and is located at the end of the visible Milky Way. The name Crux means cross in Latin. Despite its size, Crux stands out because its main stars are relatively bright. It holds great cultural significance in many countries of the Southern Hemisphere.

Orion (constellation): Orion is a famous constellation known for its prominent stars visible in the winter sky. It is one of the 88 modern constellations and was listed by the ancient astronomer Ptolemy. This constellation takes its name from Orion, a character in Greek mythology known for being a hunter.

Ursa Major: Ursa Major is a constellation in the northern sky, associated with ancient mythology. Its Latin name means "greater bear" as opposed to Ursa Minor, the lesser bear. It was among the 48 original constellations listed by Ptolemy in the 2nd century AD, based on the observations of ancient astronomers. Currently, it ranks as the third largest constellation out of the 88 recognized today.

Ursa Minor: Ursa Minor, or Little Bear, is a constellation in the northern sky. It has seven stars, four of which make up its bowl-like shape, resembling a ladle. This constellation was identified by Ptolemy and is still recognized today. Ursa Minor is important for navigation due to Polaris, the north pole star.

Planet: A planet is a large astronomical body that forms from interstellar clouds and grows through material accumulation. The Solar System has eight planets, including terrestrial and giant ones. Planets rotate around an axis and possess atmospheres, ice caps, seasons, and natural satellites. They also have magnetic fields, except for Venus and Mercury. The giant planets have planetary rings, with Saturn's rings being the most prominent.

Terrestrial planet: Terrestrial planets, also known as rocky planets, are primarily composed of silicate rocks or metals. In our Solar System, the inner planets Mercury, Venus, Earth, and Mars are considered terrestrial planets. Some astronomers include Earth's Moon, Io, and occasionally Europa, while the asteroids Pallas and Vesta are rarely considered. The term "terrestrial planet" is derived from Latin words for Earth, emphasizing their Earth-like structure. These planets are extensively studied by geologists, astronomers, and geophysicists.

Giant planet: Giant planets are significantly larger than Earth and consist mainly of low-boiling point materials. While typically not composed of solid matter, solid giant planets do exist. In our Solar System, the known giant planets are Jupiter, Saturn, Uranus, and Neptune. Additionally, numerous giant planets have been discovered beyond our Solar System orbiting different stars.

Exoplanet: An exoplanet is a planet located outside our Solar System. Possible evidence of exoplanets was observed in 1917, but the first confirmed detection occurred in 1992. Currently, there are 5,576 confirmed exoplanets in 4,113 planetary systems, with some systems having multiple planets. The James Webb Space Telescope is expected to unveil more exoplanets and provide valuable insights into their composition, environmental conditions, and potential for supporting life.

Atmosphere: An atmosphere is a layer of gas surrounding a planet, held by gravity. It remains when gravity is strong and temperature is low. Stellar atmospheres include regions above the photosphere, while cool stars may have atmospheres with compound molecules.

Ring system: A ring system is a disk composed of dust and moonlets that orbits an astronomical object, commonly found around giant planets like Saturn. It is also known as a planetary ring system.

Minor planet: A minor planet is an object in orbit around the Sun, distinct from planets and comets. They were reclassified into dwarf planets and small Solar System bodies in 2006 by the International Astronomical Union.

Asteroid: An asteroid is a minor planet without an atmosphere that orbits the inner Solar System. They come in various sizes, from meter-sized rocks to dwarf planets nearly 1000 km wide. Asteroids can be rocky, metallic, or icy in composition.

Dwarf planet: A dwarf planet is a small object orbiting the Sun, rounded by gravity but lacking the dominance of the eight classical planets. Pluto is the prototypical dwarf planet, once considered a planet before the concept changed in 2006.

Natural satellite: A natural satellite is an astronomical body that orbits a planet or other celestial object. They are commonly known as moons, like the Moon of Earth.

Comet: A comet is a small Solar System body that warms and releases gases when it approaches the Sun, creating an extended atmosphere called a coma and sometimes a tail of gas and dust. These effects are caused by solar radiation and the solar wind acting upon the comet's icy nucleus composed of ice, dust, and rocks. Comets can be seen from Earth without a telescope if they are close and bright enough, and they have been observed and recorded since ancient times.

Meteoroid: A meteoroid is a small rock or metal object in space, smaller than an asteroid. They range in size from tiny grains to objects up to one meter wide. Meteoroids are typically fragments from comets or asteroids, but some are debris ejected from the Moon or Mars due to collisions. Micrometeoroids or space dust are smaller than meteoroids.

Meteor shower: A meteor shower is when meteors appear to radiate from a single point in the night sky. They are caused by cosmic debris entering Earth's atmosphere at high speeds. Most meteors disintegrate before hitting the Earth. Intense showers, called meteor outbursts or storms, can produce over 1,000 meteors an hour. The Leonids are a notable example. There are over 900 suspected meteor showers, with about 100 well established. Viewing opportunities are available online, including NASA's daily map of active showers.

Impact event: An impact event is a collision between astronomical objects. They have physical consequences and are common in planetary systems, usually involving asteroids, comets, or meteoroids. Large impacts on terrestrial planets like Earth can have significant physical and biospheric effects, but atmospheres help reduce surface impacts. Impact craters and structures are prevalent in the Solar System, providing strong evidence of their frequency and scale.

Albedo: Albedo is the amount of sunlight that a body reflects, measured from 0 to 1.

Star: A star is a self-gravitating luminous object made of plasma. The Sun is the closest star to Earth. While many stars can be seen with the naked eye, their distance makes them appear as fixed points of light. Prominent stars are categorized into constellations and asterisms and often have proper names. Astronomers create star catalogues to identify and classify known stars. The observable universe contains an estimated 1-10 trillion trillion stars, with only about 4,000 visible to the naked eye within the Milky Way galaxy.

Variable star: A variable star is a star that exhibits changes in its brightness over time. These changes can be caused by either the star itself changing in luminosity or by something obstructing the light reaching Earth, such as an orbiting companion.

Nova: A nova is an astronomical event where a bright new star suddenly appears but then gradually fades over weeks or months. Novae are caused by specific circumstances between two stars, typically involving white dwarfs in close binary systems. There are three main sub-classes of novae: classical novae, recurrent novae, and dwarf novae. These events are categorized as cataclysmic variable stars.

Cepheid variable: A Cepheid variable is a type of star that pulsates in size and temperature. It shows predictable changes in brightness with a stable period and amplitude.

Star system: A star system consists of a few stars that orbit each other due to gravitational attraction. It is distinct from planetary systems, which include planets and other bodies. Star clusters and galaxies are examples of larger groups of stars bound by gravity.

Planetary system: A planetary system is a collection of objects that are held together by gravity and orbit around a star. It typically includes planets, but can also contain other celestial bodies like dwarf planets, asteroids, satellites, comets, and disks. The Solar System, consisting of the Sun and the objects orbiting around it, is an example of a planetary system. Similar systems outside our Solar System are referred to as exoplanetary systems.

Binary star: A binary star is a system of two stars that orbit around each other. Some binary stars can appear as a single star in the sky, but can be resolved using a telescope. Binary stars can have long orbital periods and their orbits may be uncertain. They can be detected through spectroscopy or astrometry. If a binary star is in a plane along our line of sight, the components can eclipse and transit each other, known as eclipsing binaries. Some binary stars also change brightness as they orbit, called photometric binaries.

Star cluster: Star clusters are groups of stars held together by gravity. There are two main types: globular clusters, which are tight groups of old stars, and open clusters, which are looser groups of younger stars. Open clusters can be disrupted over time by giant molecular clouds, but the stars within them continue to move in the same general direction and are called stellar associations or moving groups.

White dwarf: A white dwarf is a dense remnant of a star's core, made up of electron-degenerate matter. It has a similar mass to the Sun but has a much smaller volume, similar to the Earth's. White dwarf's low brightness is due to the release of residual thermal energy, as no fusion occurs. The closest known white dwarf is Sirius B, located 8.6 light-years away. Among the hundred star systems nearest to the Sun, there are believed to be eight white dwarfs. The dimness of white dwarfs was initially observed in 1910, and the term "white dwarf" was coined in 1922 by Willem Luyten.

Neutron star: A neutron star is the collapsed core of a massive supergiant star. They are the smallest, densest known class of stellar objects, except for black holes. Neutron stars have a radius of about 10 kilometers and a mass of approximately 1.4 solar masses. They form from supernova explosions and gravitational collapse, reaching densities similar to atomic nuclei.

Pulsar: A pulsar is a rapidly spinning neutron star that emits beams of radiation from its magnetic poles. These beams are only visible when they face Earth, resulting in a pulsed appearance. Pulsars have dense bodies and consistent rotation, creating precise intervals between pulses. They are considered as possible sources of ultra-high-energy cosmic rays.

Stellar classification: Stellar classification is the categorization of stars based on their spectral characteristics. By analyzing the electromagnetic radiation emitted by a star through a prism or diffraction grating, its spectrum reveals distinct colors and spectral lines that correspond to specific chemical elements or molecules. These lines' strengths indicate the abundance of elements, mainly influenced by the star's photosphere temperature. A star's spectral class is a concise code summarizing its ionization state, providing an objective measure of the photosphere's temperature.

Main sequence: The main sequence is a grouping of stars that form a distinct band on plots of stellar color versus brightness. These stars, also known as dwarf stars, represent the most common type of stars in the universe, including the Sun. Their position on this band provides insights into their physical properties and their progression through various stages of their life cycle. Hertzsprung–Russell diagrams, named after Hertzsprung and Russell, are used to depict this relationship between color and brightness.

Giant star: A giant star is larger and brighter than a main-sequence star with the same surface temperature. They are located above the main sequence on the Hertzsprung–Russell diagram and are classified as luminosity classes II and III. The terms giant and dwarf were coined by Ejnar Hertzsprung in 1905 for stars with different luminosities but similar temperatures.

Red giant: A red giant is a large, luminous star in a late phase of evolution with an inflated and tenuous atmosphere. It has a low to intermediate mass and a surface temperature of around 5,000 K or lower. Red giants have a yellow-white to reddish-orange appearance and can be classified as spectral types K, M, G, S, and carbon stars.

Supergiant: Supergiants are extremely massive and luminous stars that exist in the top region of the Hertzsprung–Russell diagram. They have absolute visual magnitudes ranging between approximately -3 and -8. Supergiant stars exhibit a temperature range of 3,400 K to over 20,000 K.

Wolf–Rayet star: Wolf–Rayet stars, or WR stars, are rare stars with unique spectra displaying broad emission lines of ionised helium and highly ionised nitrogen or carbon. They exhibit high surface enhancement of heavy elements, depletion of hydrogen, and strong stellar winds. With surface temperatures ranging from 20,000 K to around 210,000 K, they are hotter than most other stars. These stars were previously known as W-type stars based on their spectral classification.

Stellar evolution: Stellar evolution refers to the process of how a star changes over time. Different star masses have varying lifetimes, from millions to trillions of years. Stars are formed from collapsing gas and dust clouds, eventually settling into a stable state known as a main-sequence star.

Star formation: Star formation is the process of dense regions collapsing within interstellar space to create stars. It involves studying the interstellar medium and giant molecular clouds as precursors, as well as protostars and young stellar objects as immediate products. It is closely connected to planet formation and also considers binary stars and the initial mass function. Star clusters and stellar associations are common outcomes.

Starburst galaxy: A starburst galaxy is characterized by an extremely high rate of star formation, exceeding that of most other galaxies. The star formation rate in these galaxies can reach 100 times or more the rate of our Milky Way. This rapid star formation depletes the galaxy's gas supply, leading to a short-lived phase in its evolution. Starburst galaxies are often associated with mergers or close encounters with other galaxies. Examples of starburst galaxies include M82, NGC 4038/NGC 4039, and IC 10.

Protostar: A protostar is a young star that acquires mass from its parent molecular cloud. This phase marks the early stage of stellar evolution, lasting approximately 500,000 years for low-mass stars. It begins when a molecular cloud fragment collapses, creating an opaque, pressure-supported core. The protostar phase concludes when the incoming gas is exhausted, resulting in a pre-main-sequence star that later contracts to become a main-sequence star through hydrogen fusion and helium production.

Stellar kinematics: Stellar kinematics is the study of star motion in space. It involves observing and measuring the movement of stars.

Stellar magnetic field: A stellar magnetic field is generated by the motion of conductive plasma within a star. This motion is driven by convection, where material physically moves to transport energy. The localized magnetic field exerts a force on the plasma, increasing pressure without density. Magnetized regions rise to the star's surface, creating starspots and coronal loops.

Stellar structure: Stellar structure models predict a star's internal makeup and behavior, encompassing elements like luminosity, color, and future evolution. Diverse classes and ages of stars exhibit distinct internal structures, influenced by elemental composition and energy transport mechanisms.

Metallicity: Metallicity refers to the amount of elements in an object that are heavier than hydrogen and helium. In astronomy, "metals" is used as a term to describe all elements except hydrogen and helium. This is different from the traditional definition of a metal. Objects with high abundances of heavier elements are called "metal-rich" in astrophysics, despite some elements being nonmetals in chemistry.

Brown dwarf: Brown dwarfs are objects that have more mass than gas giants but less than stars. They are about 13 to 80 times the mass of Jupiter and cannot sustain nuclear fusion. However, they emit light and heat from the fusion of deuterium and some can even fuse lithium.

Hertzsprung–Russell diagram: The Hertzsprung-Russell diagram is a plot comparing stars' absolute magnitudes or luminosities with their classifications or temperatures. It was independently created in 1911 and 1913 by Hertzsprung and Russell, respectively. This diagram played a significant role in advancing our understanding of stellar evolution.

Planetary nebula: A planetary nebula is a glowing shell of ionized gas that is emitted from red giant stars during the later stages of their lives. It is a type of emission nebula found in space.

Supernova: A supernova is a massive explosion that occurs when a star collapses or undergoes runaway nuclear fusion. It results in the formation of a neutron star or black hole, or the complete destruction of the star. Supernovae exhibit extremely bright optical luminosity, comparable to entire galaxies, before gradually fading over weeks or months.

Gamma-ray burst: Gamma-ray bursts (GRBs) are extremely powerful explosions observed in distant galaxies. They are the most energetic events since the Big Bang, emitting gamma rays and a longer-lasting afterglow. GRBs can last from milliseconds to hours and are the most powerful class of explosions in the universe, according to NASA.

Outer space: Outer space is the vast expanse beyond celestial bodies and their atmosphere. It is not empty, but instead contains a low density of particles such as hydrogen, helium, electromagnetic radiation, magnetic fields, neutrinos, dust, and cosmic rays. Its baseline temperature, set by the background radiation from the Big Bang, is 2.7 kelvins.

Galaxy: A galaxy is a system of stars, gas, dust, and dark matter held together by gravity. The term comes from the Greek word for "milky," referring to our own Milky Way galaxy. Galaxies vary greatly in size, from dwarfs to supergiants with trillions of stars. Most of a galaxy's mass is in the form of invisible dark matter. Supermassive black holes are frequently found at galaxy centers.

Galaxy formation and evolution: The study of how galaxies form and change over time, including the variety of structures observed. This occurs through processes like clustering, merging, and the accumulation of mass. It is hypothesized to be influenced by quantum fluctuations after the Big Bang. Hydrodynamics simulation is commonly used to study this.

Black hole: A black hole is a region in space where gravity is incredibly strong, not allowing anything, including light, to escape. It forms when a mass becomes compact enough, deforming spacetime. The boundary where nothing can escape is called the event horizon. Black holes have no detectable features and act like ideal black bodies, reflecting no light. Additionally, they emit Hawking radiation with a temperature inversely proportional to their mass, making direct observation practically impossible.

Supermassive black hole: A supermassive black hole is the largest kind of black hole, with a mass millions to billions of times that of the Sun. These objects are formed from the gravitational collapse of massive stars and are found at the center of most large galaxies, including our own Milky Way. They are incredibly powerful, attracting and devouring interstellar gas, which fuels the intense energy emissions from active galactic nuclei and quasars.

Event horizon: An event horizon is a boundary in astrophysics that marks the point beyond which events are no longer observable to an observer. Coined by Wolfgang Rindler in the 1950s.

Interstellar medium: The interstellar medium (ISM) is the matter and radiation present in the space between star systems in a galaxy. It consists of gas in different forms, dust, cosmic rays, and electromagnetic radiation. The ISM fills the space between galaxies as well. Despite low atomic density, collisions between particles are frequent, making it behave like a gas and responding to pressure forces.

Nebula: A nebula is a glowing part of the space between stars, consisting of hydrogen, cosmic dust, and other materials. Nebulae often form new stars and planetary systems by gathering gas and dust into dense regions. These regions eventually become stars, while the remaining material forms planets and other celestial objects.

Dark nebula: A dark nebula is a dense interstellar cloud that blocks visible light from objects behind it. It is made up of interstellar dust grains found in the coldest parts of molecular clouds. Clusters of dark nebulae are linked to Giant Molecular Clouds, while small isolated ones are known as Bok globules. Radio waves and infrared astronomy are used to observe objects hidden by dark nebulae.

Molecular cloud: A molecular cloud, also known as a stellar nursery, is a dense and large interstellar cloud that allows for the formation of molecules, primarily molecular hydrogen (H2), and the creation of absorption nebulae and H II regions. It is distinct from other regions in the interstellar medium that primarily contain ionized gas.

H II region: An H II region is a ionized region of interstellar hydrogen found in molecular clouds. It ranges in size from one to hundreds of light years and has a density of about a million particles per cubic centimeter. The Orion Nebula, discovered in 1610, is an example of an H II region.

Cosmic ray: Cosmic rays are high-energy particles traveling close to the speed of light. They come from various sources, including the Sun, our galaxy, and distant galaxies. When they collide with Earth's atmosphere, cosmic rays create showers of secondary particles. While some particles reach the surface, most are redirected away from Earth by the magnetosphere or heliosphere.

Galaxy groups and clusters: Galaxy groups and clusters are the densest part of the Universe's large-scale structure. They are the largest gravitationally bound objects formed during cosmic structure formation. These clusters, formed relatively recently, contain ten to thousands of galaxies and are associated with larger non-gravitationally bound superclusters.

Supercluster: A supercluster is a massive aggregation of smaller galaxy clusters or groups, making them some of the largest structures in the universe. The Milky Way is part of the Local Group, which belongs to the Virgo Supercluster, itself part of the even larger Laniakea Supercluster in the Pisces–Cetus Complex. Superclusters expand with the Hubble expansion due to their large size and low density. It is estimated that there are about 10 million superclusters in the observable universe.

Void (astronomy): Cosmic voids are vast spaces in astronomy that have very few or no galaxies. Most galaxies are not found in voids because they are gravitationally bound together in large structures called galaxy filaments. The evolution of void regions differs from that of the Universe as a whole, with a long stage dominated by curvature that hinders the formation of galaxy clusters and massive galaxies. Even though voids contain over 15% of the average matter density of the Universe, they appear nearly empty to an observer.

Galaxy morphological classification: Galaxy morphological classification is a method astronomers use to categorize galaxies based on their visual appearance. The most well-known system is the Hubble sequence, developed by Edwin Hubble and expanded by Gérard de Vaucouleurs and Allan Sandage. Today, computational methods and physical morphology play a significant role in classifying galaxies.

Spiral galaxy: A spiral galaxy is a type of galaxy that was first described by Edwin Hubble in 1936. It consists of a flat disk containing stars, gas, and dust, with a central concentration of stars called the bulge. Spiral galaxies also have a fainter halo of stars around them, including globular clusters.

Barred spiral galaxy: A barred spiral galaxy is a type of spiral galaxy that has a bar-shaped structure made up of stars at its center. These bars are present in about two thirds of all spiral galaxies in our local universe and have an impact on the movement of stars and interstellar gas, as well as the formation of spiral arms. Our own Milky Way Galaxy, home to the Solar System, falls into this category.

Elliptical galaxy: An elliptical galaxy is a smooth, ellipsoidal-shaped galaxy with few distinct features. It is one of the four main types of galaxies described by Edwin Hubble. Along with spiral and lenticular galaxies, they are part of the "early-type" galaxy population.

Lenticular galaxy: A lenticular galaxy is a type of galaxy that lies between an elliptical and a spiral galaxy. It has a large-scale disc but lacks spiral arms. Lenticular galaxies have little ongoing star formation due to the depletion of interstellar matter, although they may retain dust in their discs. They are primarily composed of aging stars. Despite their morphological differences, lenticular and elliptical galaxies share common properties and can be considered early-type galaxies undergoing passive evolution. They are connected to S0 galaxies through ES galaxies that possess intermediate-scale discs.

Irregular galaxy: An irregular galaxy lacks a defined shape unlike the spiral or elliptical galaxies. They are not classified under the Hubble sequence and appear chaotic with no central bulge or spiral arms.

Dwarf galaxy: A dwarf galaxy is a small galaxy with up to several billion stars, compared to the Milky Way's 200-400 billion stars. The Large Magellanic Cloud, sometimes classified as a dwarf galaxy, orbits the Milky Way and contains over 30 billion stars. Dwarf galaxies' formation and activity are influenced by interactions with larger galaxies. Astronomers categorize dwarf galaxies based on shape and composition.

Active galactic nucleus: An active galactic nucleus (AGN) is a compact region in the center of a galaxy that emits a significant amount of energy across various wavelengths. This excess radiation, not produced by stars, is likely caused by a supermassive black hole accreting matter. AGNs are found in active galaxies.

Seyfert galaxy: Seyfert galaxies are active galaxies, similar to quasars, with bright nuclei and high-ionization emission lines in their spectra. Unlike quasars, Seyfert galaxies have detectable host galaxies.

Radio galaxy: A radio galaxy is a galaxy that emits giant regions of radio waves extending beyond its visible structure. These radio emissions are powered by jets from its active galactic nucleus, with luminosities reaching up to 1039 W. The radio emission is a result of the synchrotron process, and the observed structure is influenced by the interaction between twin jets and the surrounding medium, affected by relativistic beaming. These galaxies are typically large elliptical galaxies and are valuable for studying cosmology due to their detectability at large distances. Their effects on the intergalactic medium, particularly in galaxy groups and clusters, have been extensively researched.

Quasar: A quasar, also known as a quasi-stellar object (QSO), is an extremely bright active galactic nucleus (AGN). It is powered by a supermassive black hole, surrounded by a gaseous accretion disc. The disc releases enormous amounts of energy in the form of electromagnetic radiation. Most quasars have luminosities thousands of times greater than the Milky Way. They are a subclass of AGN and their redshifts are cosmological in origin.

Blazar: A blazar is an active galactic nucleus that appears bright due to a relativistic jet pointed towards Earth. It emits high-energy gamma ray photons and shows rapid fluctuations in brightness. Some blazar jets exhibit superluminal motion as they travel at nearly the speed of light towards observers.

Physical cosmology: Physical cosmology is a branch of science that studies cosmological models to understand the structure, dynamics, origin, evolution, and fate of the universe. It emerged from the Copernican principle and Newtonian mechanics, which helped understand the laws governing celestial bodies.

Universe: The universe is all of space and time, including everything that exists from sub-atomic particles to galaxies. It emerged from the Big Bang about 13.787 billion years ago and has been expanding ever since. The observable universe is about 93 billion light-years in diameter, but the size of the entire universe is unknown.

Observable universe: The observable universe is a region of space containing all matter that can be seen from Earth or its telescopes. It forms a spherical shape and includes objects whose light has reached Earth since the expansion of the universe began. It was initially speculated to have 2 trillion galaxies, but recent data suggests several hundred billion. This spherical region is centered on the observer, and every location in the universe has its own observable universe which can overlap with Earth's perspective.

Chronology of the universe: The chronology of the universe is a description of its history and future based on Big Bang cosmology. It provides insights into the formation and development of the universe since its birth.

Big Bang: The Big Bang is a theory that explains the expansion of the universe from a dense and hot initial state. Proposed by Georges Lemaître in 1927, it offers comprehensive explanations for various observed phenomena. These include the abundance of light elements, the cosmic microwave background radiation, and the large-scale structure of the universe. Cosmic inflation helps explain the overall uniformity of the universe. However, the earliest conditions of the Big Bang lack a widely accepted theory of quantum gravity.

Inflation (cosmology): Cosmic inflation, or just inflation, is a theory in cosmology that explains the rapid expansion of space in the early universe. This inflationary epoch lasted from 10−36 seconds to between 10−33 and 10−32 seconds after the Big Bang. After inflation, the universe continued to expand at a slower rate until the acceleration caused by dark energy, which occurred over 7.7 billion years later.

Ultimate fate of the universe: The ultimate fate of the universe refers to the possible outcomes of its evolution. This topic in physical cosmology explores different scenarios and evaluates them based on observational evidence. It is a valid cosmological question that goes beyond mythological or theological beliefs. Scientists propose various hypotheses, including finite and infinite durations for the universe, to explain its beginning and future.

Cosmic microwave background: The cosmic microwave background is a faint glow of microwave radiation that fills all of space in the observable universe. It provides valuable information about the early universe and was accidentally discovered in 1965 by radio astronomers Arno Penzias and Robert Wilson.

Dark matter: Dark matter is a hypothetical form of matter in astronomy that doesn't interact with light or electromagnetic fields. It is inferred from gravitational effects that cannot be explained by general relativity with the visible matter alone. These effects are observed in the formation and evolution of galaxies, gravitational lensing, galactic collisions, galaxy cluster motion, and cosmic microwave background anisotropies.

Dark energy: Dark energy is an unknown energy that drives the expanding universe. It makes up 68% of the universe's total energy, while dark matter and ordinary matter contribute 26% and 5%. Despite its low density, dark energy dominates the universe due to its uniformity throughout space.

Cosmological constant: The cosmological constant, also known as Einstein's cosmological constant, is a coefficient that Einstein added temporarily to his equations of general relativity but later removed. It was then rediscovered as the energy density of space or vacuum energy, which is related to the concept of dark energy in quantum mechanics.

Redshift: Redshift refers to the increase in wavelength and decrease in frequency and energy of electromagnetic radiation. This change is opposite to blueshift. Redshift is caused by relative motion of sources and gravitational potentials. It is seen in all distant light sources, indicating the expansion of the universe.

Hubble's law: Hubble's Law states that galaxies are receding from us at speeds proportional to their distance, meaning the farther the galaxy, the faster it is moving away. The velocity of galaxies is found through their redshift, a shift of emitted light towards the red end of the visible spectrum.

Chemistry: Chemistry is the study of matter, its properties, behavior, composition, and changes during reactions. It explores elements, compounds, atoms, molecules, ions, and chemical bonds.

Alchemy: Alchemy is an ancient, philosophical tradition practiced in China, India, the Muslim world, and Europe. It emerged in Greco-Roman Egypt during the first few centuries AD.

Analytical chemistry: Analytical chemistry employs techniques to separate, identify, and measure substances in order to determine their composition. This field covers both qualitative analysis for identifying substances and quantitative analysis for determining their amount or concentration.

Environmental chemistry: Environmental chemistry is the study of chemical and biochemical processes in natural environments, with a focus on understanding the sources, reactions, transport, effects, and fates of chemical species in the air, soil, and water. It examines the impact of both human and biological activities on these processes. This interdisciplinary field incorporates atmospheric, aquatic, and soil chemistry, relying heavily on analytical chemistry and its connection to environmental and other scientific disciplines.

Inorganic chemistry: Inorganic chemistry focuses on the behavior and synthesis of compounds that are not carbon-based. While it overlaps with organic chemistry in organometallic chemistry, it has diverse applications in areas like catalysis, materials science, pigments, medications, fuels, and agriculture.

Organic chemistry: Organic chemistry is a branch of chemistry that studies the structure, properties, and reactions of compounds containing carbon atoms. It includes analyzing their structural formula, physical and chemical properties, and behavior. This field involves synthesizing natural products, drugs, and polymers, as well as studying individual organic molecules in labs and through theoretical research.

Medicinal chemistry: Medicinal chemistry combines chemistry and pharmacy to create and improve drugs. It involves discovering and creating new chemicals for medical purposes, as well as studying the properties of existing drugs and their effects on the body.

Organometallic chemistry: Organometallic chemistry is the study of compounds that have at least one bond between a carbon atom of an organic molecule and a metal. It includes alkali, alkaline earth, and transition metals, as well as metalloids. Compounds with bonds to inorganic carbon are also considered organometallic. Some related compounds, such as metal hydrides and metal phosphine complexes, are often discussed in relation to organometallic compounds. Metalorganic compounds are similar but lack direct metal-carbon bonds, instead having organic ligands. Organometallic chemistry combines aspects of inorganic and organic chemistry.

Physical chemistry: Physical chemistry is the branch of science that investigates chemical systems using the principles of physics. It analyzes both large and small-scale phenomena, exploring motion, energy, force, time, thermodynamics, quantum chemistry, statistical mechanics, dynamics, and chemical equilibria.

Electrochemistry: Electrochemistry studies the connection between electrical potential difference and chemical change. It involves electron movement between electrodes through an electrolyte as reactions take place.

Nuclear chemistry: Nuclear chemistry deals with radioactivity, nuclear processes, and transformations in atomic nuclei. It encompasses nuclear transmutation and properties of nuclei.

Photochemistry: Photochemistry is a branch of chemistry that studies the effects of light on chemicals. It refers to reactions triggered by the absorption of ultraviolet, visible, or infrared radiation.

Quantum chemistry: Quantum chemistry applies quantum mechanics to study chemical systems, calculating electronic contributions to properties of molecules, materials, and solutions. It uses approximations to make computations practical while capturing important information about wave functions and observable properties. It also investigates quantum effects on molecular dynamics and chemical kinetics.

Materials science: Materials science is an interdisciplinary field that researches and discovers new materials. Materials engineering, on the other hand, focuses on finding practical applications for these materials in various industries.

Polymer chemistry: Polymer chemistry is a branch of chemistry that studies the structures, synthesis, and properties of polymers and macromolecules. It connects various sub-disciplines like organic, analytical, and physical chemistry. Polymer chemistry encompasses both synthetic and organic compositions, playing a crucial role in materials like plastics and rubbers. It is closely related to polymer science, nanotechnology, polymer physics, and polymer engineering.

Chemical bond: A chemical bond is a lasting attraction between atoms or ions that enables the formation of molecules and other structures. It can be formed through the electrostatic force between oppositely charged ions (ionic bond) or the sharing of electrons (covalent bond). Chemical bonds can be strong (covalent, ionic, metallic) or weak (dipole-dipole, London dispersion, hydrogen bonding).

Covalent bond: A covalent bond is a chemical bond formed when atoms share electrons, resulting in stable attractive and repulsive forces. It allows atoms to attain a stable electronic configuration, commonly seen in organic chemistry.

Hydrogen bond: A hydrogen bond is an electrostatic attraction between a hydrogen atom and an electronegative atom with lone pair of electrons. It is represented as Dn−H···Ac, where Dn is the donor atom and Ac is the acceptor atom. The most common atoms involved are nitrogen, oxygen, and fluorine.

Ionic bonding: Ionic bonding is a chemical bond formed through the attraction between oppositely charged ions. It occurs between atoms with significantly different electronegativities, leading to the creation of ionic compounds. This type of bonding involves the transfer of electrons, resulting in negatively charged ions for electron gain and positively charged ions for electron loss. The cation is typically a metal atom, while the anion is a nonmetal atom. However, ions can be more complex, like molecular ions. In simple terms, ionic bonds occur when electrons are transferred from a metal to a non-metal, allowing both atoms to achieve a full outer electron shell.

Metallic bonding: Metallic bonding is a chemical bonding between conduction electrons and metal ions, resulting in the sharing of free electrons among positively charged ions. This type of bonding explains various physical properties of metals including strength, ductility, thermal and electrical conductivity, opacity, and lustre.

Mixture: In chemistry, a mixture is a combination of different substances without chemical bonding. It retains the identities of its components and can exist in the form of solutions, suspensions, or colloids.

Dispersion (chemistry): Dispersion in chemistry refers to a system where particles of one substance are spread throughout another substance, which can be in the same or different state.

Colloid: A colloid is a mixture of insoluble particles suspended in another substance. It can include liquids, aerosols, and gels. The overall mixture is called a colloidal suspension. The dispersed phase particles have a size range of approximately 1 nanometre to 1 micrometre.

Aerosol: An aerosol is a mixture of fine solid particles or liquid droplets suspended in air or gas. It can be natural or caused by human activity. Examples of natural aerosols include fog, dust, and steam, while anthropogenic aerosols include pollutants, perfume, and medical treatments. Inhaling the contents of a vape pen or e-cigarette means inhaling an anthropogenic aerosol.

Emulsion: An emulsion is a mixture of normally immiscible liquids, where one liquid is dispersed in the other. It is part of a broader category of two-phase systems known as colloids. Emulsions are used to describe mixtures with both dispersed and continuous liquid phases. Examples of emulsions include vinaigrettes, homogenized milk, biomolecular condensates, and certain cutting fluids.

Foam: Foam refers to materials comprising gas trapped in a liquid or solid.

Gel: A gel is a semi-solid substance that can vary in softness and hardness. It is a dilute cross-linked system that does not flow in a steady state but allows diffusion of liquid. A gel is a soft or solid-like material composed of two or more components, one being a significant amount of liquid.

Solution (chemistry): A solution is a homogeneous mixture of substances where a solute is dissolved in a solvent. The solvent particles separate the solute particles and surround them, allowing them to move freely. The process of mixing a solution involves specific interactions due to polarity. Concentration is an important parameter, measuring the amount of solute in the solution. An aqueous solution refers to a solution where water is one of the solvents.

Suspension (chemistry): A suspension in chemistry refers to a mixture of a fluid and visible solid particles that are large enough to settle. The particles must be larger than one micrometer and the mixture is classified as a suspension until the particles have settled.

Periodic table: The Periodic Table is a fundamental tool in chemistry, used in various sciences, that organizes the chemical elements into rows and columns. It represents the periodic law, showing that elements' properties recur as atomic numbers increase. The table is divided into four blocks, and elements within the same group exhibit similar chemical characteristics.

Atomic mass: The atomic mass is the mass of an atom, typically expressed in daltons. It is the sum of the masses of protons and neutrons in the nucleus, while electrons and nuclear binding energy have minor contributions. The atomic mass constant allows for conversion between kilograms and daltons.

Atomic number: The atomic number (Z) is the charge number of an atomic nucleus, representing the number of protons in an element. It uniquely identifies a chemical element and is equal to the number of electrons in an uncharged atom.

Metal: Metal is a lustrous material that conducts electricity and heat well. It is also ductile and malleable due to the metallic bond between its atoms or molecules.

Metalloid: A metalloid is a chemical element exhibiting properties that are a mix of both metals and nonmetals. Although there is no universally accepted definition or consensus on which elements are metalloids, the term continues to be used in the field of chemistry.

Chemical reaction: A chemical reaction is a process that transforms one set of substances into another. It typically involves changes in electron positions, forming and breaking chemical bonds. Nuclear chemistry deals with reactions of unstable and radioactive elements, where both electronic and nuclear changes happen.

Catalysis: Catalysis is when a substance called a catalyst increases the speed of a chemical reaction. The catalyst is not used up in the reaction and remains unchanged. Even small amounts of catalyst can be effective if it recycles quickly. Factors such as mixing, surface area, and temperature influence the rate of the reaction. Catalysts react with reactants to form intermediates and regenerate the catalyst in the process.

Chemical equation: A chemical equation is a symbolic representation of a chemical reaction using formulas and symbols. It shows reactants on the left and products on the right, with a plus sign between them. The arrow indicates the direction of the reaction. The coefficients represent stoichiometric numbers, and the first chemical equation was diagrammed by Jean Beguin in 1615.

Chemical equilibrium: Chemical equilibrium is when the concentrations of reactants and products in a reaction remain constant over time, resulting in no observable changes in the system's properties. This state occurs when the forward and reverse reactions occur at the same rate, leading to no net changes in reactant and product concentrations. It is referred to as dynamic equilibrium.

Chemical synthesis: Chemical synthesis is the process of artificially creating chemical reactions to obtain one or multiple products. It involves physical and chemical manipulations, often through multiple reactions. This reproducible and reliable process is commonly used in modern laboratories.

Chemical substance: A chemical substance is a distinct form of matter with consistent chemical composition and unique properties. It can be a single element or a compound. When multiple substances are combined without reacting, they form a mixture. If a mixture is separated to isolate a specific substance, it is considered chemically pure.

Amount of substance: The amount of substance is a measure of the number of molecules, atoms, or ions in a sample of matter. It is represented by the symbol 'n' and is calculated by dividing the number of elementary entities (N) by the Avogadro constant (NA). The unit for amount of substance is the mole (mol). The Avogadro constant was recently defined as exactly 6.02214076×1023 mol−1. This measure is often referred to as the chemical amount or the "number of moles" in a sample of matter.

Chemical element: A chemical element is a substance that cannot be broken down into other substances. It consists of atoms, with the number of protons in an atom's nucleus determining the element's atomic number. Elements can combine to form molecules, while nuclear reactions can transform atoms into different elements.

Allotropy: Allotropy refers to the ability of certain elements to exist in multiple forms known as allotropes. These allotropes are distinct structural variations of the element, with different bonding arrangements. Carbon, for instance, exhibits allotropy with allotropes such as diamond, graphite, graphene, and fullerenes.

Chemical formula: A chemical formula is a way of showing the proportions of atoms in a compound using symbols and numbers. It is not a chemical name and does not contain words. Chemical formulas can give a basic idea of the structure of a compound but are not as powerful as chemical names or full structural formulas.

Chemical compound: A chemical compound is composed of identical molecules containing atoms from different elements. It is held together by chemical bonds and can undergo a chemical reaction to transform into a different substance by breaking and forming new bonds. Molecules consisting of only one element are not compounds.

Molecule: A molecule is formed when two or more atoms are bonded together by attractive forces called chemical bonds. It can include ions but is often used to refer to polyatomic ions in quantum physics, organic chemistry, and biochemistry.

Inorganic compound: An inorganic compound is a chemical compound without carbon-hydrogen bonds, also known as an organic compound. It is studied in the subfield of chemistry called inorganic chemistry.

Organic compound: An organic compound is a chemical compound that generally contains carbon–hydrogen or carbon–carbon bonds. However, some consider any compound with carbon to be organic. For instance, alkanes and their derivatives are universally recognized as organic, while certain compounds involving carbon, nitrogen, and oxygen may be considered inorganic.

Hydrogen: Hydrogen is the lightest chemical element, a colorless, odorless, and highly combustible gas. It is the most abundant substance in the universe, constituting approximately 75% of all matter. Stars like the Sun primarily consist of hydrogen in its plasma state. On Earth, hydrogen exists in molecular forms like water and organic compounds. Its most common isotope has one proton, one electron, and no neutrons.

Helium: Helium is a chemical element with atomic number 2 and symbol He. It is a colorless, odorless, and tasteless gas. As the first noble gas in the periodic table, it is inert and monatomic. Helium has the lowest boiling point and is the second-lightest element in the universe, making it abundant and constituting about 24% of the total elemental mass. Its abundance is similar in the Sun and Jupiter. Helium is a product of both nuclear fusion and radioactive decay, and its most common isotope is helium-4, formed during the Big Bang and through fusion in stars.

Alkali metal: Alkali metals are a group of chemical elements including lithium, sodium, potassium, rubidium, caesium, and francium. They are found in group 1 of the periodic table, sharing similar properties due to their outermost electron being in an s-orbital. They display consistent trends in properties and are known as the lithium family.

Lithium: Lithium is a chemical element with symbol Li and atomic number 3. It is a highly reactive and flammable alkali metal with a soft, silvery-white appearance. Being the least dense metal and solid element, it quickly corrodes in air. Though not occurring freely in nature, lithium is found in pegmatitic minerals and can be obtained from ocean water or brines. It is commonly isolated electrolytically from a mixture of lithium chloride and potassium chloride.

Sodium: Sodium is a highly reactive, silvery-white metal found in group 1 of the periodic table. It is symbolized as Na and has an atomic number of 11. Sodium is primarily obtained from compounds as it doesn't occur naturally as a free metal. It is the sixth most abundant element in the Earth's crust and is present in minerals like feldspars, sodalite, and halite. Sodium and chlorine are the most commonly dissolved elements in the oceans due to water leaching from minerals.

Potassium: Potassium (K) is a soft, silvery white metal that reacts rapidly with oxygen and water. It was first isolated from plant ashes, hence its name. As one of the alkali metals, it has a single outer electron that is easily removed, creating a positive ion. Potassium occurs naturally in ionic salts and can be found in seawater and minerals like orthoclase. When burned, it produces a distinct lilac-colored flame.

Rubidium: Rubidium is a soft, whitish-grey alkali metal with atomic number 37. It is the first alkali metal to have a density higher than water. Natural rubidium consists of two isotopes – 72% stable 85Rb and 28% slightly radioactive 87Rb, with a half-life of 48.8 billion years.

Caesium: Caesium is a soft, silvery-golden alkali metal with symbol Cs and atomic number 55. It is one of the few elemental metals that are liquid at or near room temperature. Caesium has similar properties to rubidium and potassium, but is pyrophoric and reacts with water even at extremely low temperatures. It is the least electronegative element and has just one stable isotope, caesium-133. Mined mostly from pollucite, it is also extracted as caesium-137 from nuclear reactor waste. Caesium has the largest atomic radius among all elements, measuring approximately 260 picometers.

Francium: Francium (Fr) is a highly radioactive chemical element with atomic number 87. It is the second-most electropositive element and the second rarest naturally occurring element. Its most stable isotope, francium-223, has a short half-life of 22 minutes. Francium's isotopes decay into astatine, radium, and radon. With an electronic structure of [Rn] 7s1, it is classified as an alkali metal.

Alkaline earth metal: The alkaline earth metals, which include beryllium, magnesium, calcium, strontium, barium, and radium, are a group of six elements in the periodic table. These metals share common characteristics: they are shiny, reactive, and have a silvery-white appearance at standard temperature and pressure.

Beryllium: Beryllium is a steel-gray alkaline earth metal with atomic number 4. It is strong, lightweight, and brittle. Occurring naturally in minerals, it is rare and found in gemstones like beryl. Beryllium is formed through cosmic reactions and depleted in stars. Constituting only 0.0004% of Earth's crust, its production involves a challenging extraction process from beryl.

Magnesium: Magnesium is a chemical element with the symbol Mg and atomic number 12. It is a shiny gray metal with low density, low melting point, and high reactivity. Found only in combination with other elements, magnesium typically has an oxidation state of +2. It reacts with air to form magnesium oxide, preventing corrosion. When ignited, it produces a bright white light. Electrolysis of magnesium salts obtained from brine is the main method of obtaining the metal. Magnesium is less dense than aluminium and is commonly used in strong and lightweight alloys alongside aluminium.

Calcium: Calcium (Ca) is a reactive alkaline earth metal with atomic number 20. It forms a dark oxide-nitride layer when exposed to air. Similar to strontium and barium, calcium's properties are both physical and chemical. It ranks as the fifth most abundant element in the Earth's crust and the third most abundant metal. The main calcium compound is calcium carbonate, found in limestone and fossils. The name calcium comes from the Latin word for lime, derived from heating limestone.

Strontium: Strontium (Sr) is a soft and reactive alkaline earth metal. It resembles calcium and barium in terms of physical and chemical properties. When exposed to air, it forms a dark oxide layer. Strontium is primarily found in the minerals celestine and strontianite and is commonly mined from these sources.

Barium: Barium (Ba) is a soft alkaline earth metal, classified as the fifth element in group 2. It has an atomic number of 56 and is highly reactive. This chemical element is never found freely in nature.

Radium: Radium is a highly radioactive chemical element (symbol Ra, atomic number 88) found in group 2 of the periodic table. It reacts with nitrogen to form a black layer of radium nitride and emits ionizing radiation during decay, causing radioluminescence. The most stable isotope, radium-226, has a half-life of 1,600 years. Radium can excite fluorescent chemicals.

Boron: Boron is a chemical element with symbol B and atomic number 5. It is a brittle, lustrous metalloid that can exist in crystalline or amorphous form. Being the lightest element in its group, it has three valence electrons, allowing it to form various compounds like boric acid and sodium borate. Boron is also known for its ultra-hard crystals like boron carbide and boron nitride.

Aluminium: Aluminium (symbol Al, atomic number 13) is a chemical element with lower density than other common metals, like steel. It forms a protective oxide layer when exposed to air and visually resembles silver. Aluminium is soft, nonmagnetic, and has a stable isotope (27Al) that is highly abundant. Additionally, it is the twelfth-most common element in the universe and is used in radiometric dating due to the radioactivity of 26Al.

Gallium: Gallium is a chemical element with symbol Ga and atomic number 31. It was discovered in 1875 by French chemist Paul-Émile Lecoq de Boisbaudran. Gallium belongs to group 13 of the periodic table and shares similarities with other metals in the group.

Indium: Indium is a soft, silvery-white chemical element with atomic number 49. It shares similarities with gallium and thallium and was discovered in 1863 through spectroscopic methods. It is named after the indigo blue line in its spectrum.

Thallium: Thallium (Tl) is a gray post-transition metal with atomic number 81. It is not naturally occurring and resembles tin, but discolors in air. Discovered independently in 1861 by William Crookes and Claude-Auguste Lamy using flame spectroscopy, thallium produces a notable green spectral line. Its name, derived from Greek, means "green shoot" or "twig." Lamy isolated it by electrolysis, while Crookes used precipitation and melting. Crookes showcased the thallium powder, obtained by zinc precipitation, at the international exhibition in 1862.

Carbon: Carbon (C) is a chemical element found in Earth's crust, comprising around 0.025 percent of it. It belongs to group 14 of the periodic table and has atomic number 6. Carbon is nonmetallic and tetravalent, allowing it to form up to four covalent bonds. It occurs naturally in three isotopes, with 12C and 13C being stable and 14C being a radioactive isotope. Carbon, one of the oldest known elements, has been used since antiquity.

Carbon nanotube: Carbon nanotubes (CNTs) are nanometer-sized tubes made entirely of carbon, classified as an allotrope.

Diamond: Diamond is a solid form of carbon with a unique crystal structure. It is incredibly hard and has high thermal conductivity. While graphite is the stable form of carbon, diamond is metastable and converts to graphite at a slow rate. Its hardness and conductivity make it valuable in industrial applications like cutting tools and diamond anvil cells for high-pressure experiments.

Graphite: Graphite is a stable form of carbon, consisting of stacked graphene layers. It occurs naturally and is consumed widely for pencils, lubricants, and electrodes. It is an excellent conductor of heat and electricity, and can convert to diamond under extreme pressures and temperatures.

Silicon: Silicon is a chemical element with symbol Si and atomic number 14. It is a hard, brittle crystalline solid with a blue-grey metallic luster. Being a metalloid and semiconductor, it is a member of group 14 in the periodic table. Silicon is relatively unreactive and finds its place in the middle, with carbon above it and germanium, tin, lead, and flerovium below it.

Germanium: Germanium is a chemical element (Ge) with atomic number 32. It is lustrous, hard-brittle, and grayish-white, resembling silicon. As a metalloid in the carbon group, germanium exhibits chemical similarities to silicon and tin. It naturally reacts with oxygen and forms complexes, much like silicon.

Tin: Tin is a soft, silvery metal with symbol Sn and atomic number 50. It can be easily cut and bent by hand. When tin is bent, a distinct "tin cry" sound can be heard due to twinning in its crystal structure. This characteristic is shared by indium, cadmium, zinc, and mercury when they are solid.

Lead: Lead is a chemical element with symbol Pb and atomic number 82. It is a dense, heavy metal that is soft and malleable, with a low melting point. It has a shiny gray appearance that tarnishes to a dull gray when exposed to air. Lead is highly toxic, particularly to children, even in small amounts. It also has the highest atomic number of any stable element and plays a significant role in nuclear decay chains.

Nitrogen: Nitrogen (N) is a nonmetal with atomic number 7 and symbol N. It is the lightest member of group 15, known as the pnictogens. Nitrogen is highly abundant in the universe, ranking seventh in total abundance in the Milky Way and the Solar System. In its diatomic form, as N2, it is a colorless and odorless gas that constitutes approximately 78% of Earth's atmosphere, making it the most abundant uncombined element in the air. However, nitrogen is relatively scarce in solid parts of the Earth due to its volatility.

Phosphorus: Phosphorus (P) is a chemical element with atomic number 15. It exists in two main forms, white phosphorus and red phosphorus. Being highly reactive, it is not found freely in nature. Phosphorus is only present in the Earth's crust in small concentrations, around one gram per kilogram. In minerals, it typically appears as phosphate.

Arsenic: Arsenic is a chemical element (symbol As, atomic number 33) found in minerals, often combined with sulfur and metals. It is a metalloid with multiple allotropes, but the grey metallic form is industrially significant.

Antimony: Antimony is a chemical element (symbol Sb, atomic number 51), occurring as a lustrous gray metalloid. It is primarily found as the sulfide mineral stibnite and has been used in medicine and cosmetics since ancient times. Known by the Arabic name kohl, Antimony has a documented history dating back to 1540 by Vannoccio Biringuccio.

Bismuth: Bismuth is a chemical element with symbol Bi and atomic number 83. It is a post-transition metal, occurring naturally in elemental form. It has similar properties to arsenic and antimony. Its sulfide and oxide forms are commercially valuable. Bismuth has a silvery-white color but can appear iridescent when oxidized. It is an extremely brittle metal and has low thermal conductivity. Bismuth is the most diamagnetic element known.

Oxygen: Oxygen, symbol O, is a chemical element found abundantly in Earth's crust and is the third-most abundant element in the universe. It is a highly reactive nonmetal that readily forms oxides with other elements and compounds. Oxygen exists as diatomic gas, O2, in the Earth's atmosphere, accounting for 20.95% of it. It is a colorless, odorless gas that plays a vital role in supporting life.

Ozone: Ozone is an inorganic molecule (O3) that is a pale blue gas with a pungent smell. It is less stable than oxygen (O2), breaking down into O2 in the lower atmosphere. Ozone forms when oxygen is exposed to UV light and electrical discharges in the Earth's atmosphere. It is found in low concentrations, with the highest concentration in the ozone layer of the stratosphere, which absorbs the majority of the Sun's UV radiation.

Sulfur: Sulfur, symbol S and atomic number 16, is a chemical element. It is abundant, nonmetallic, and forms cyclic octatomic molecules with the formula S8 under normal conditions. Elemental sulfur is a bright yellow solid at room temperature.

Selenium: Selenium, symbol Se and atomic number 34, is a chemical element. It is a nonmetal with properties similar to sulfur, tellurium, and arsenic. Discovered in 1817 by Jöns Jacob Berzelius, Selenium is rarely found in its pure form in Earth's crust.

Tellurium: Tellurium is a rare, brittle, silver-white metalloid with the chemical symbol Te and atomic number 52. It is related to selenium and sulfur and belongs to the chalcogen group. While occasionally found in its elemental crystal form, tellurium is more abundant in the Universe than on Earth. Its scarcity on Earth's crust is comparable to platinum because it formed a volatile hydride that escaped as gas during the planet's formation.

Polonium: Polonium, element 84 on the periodic table, is a rare and highly radioactive metal. It lacks stable isotopes and is chemically similar to selenium and tellurium. It shares metallic properties with thallium, lead, and bismuth. Its natural occurrence is limited to small amounts of polonium-210 in uranium ores. Polonium-209, with a half-life of 124 years, is difficult to produce. It is typically obtained by neutron irradiation of bismuth. Due to its intense radioactivity, its chemistry has been primarily studied on a small scale.

Halogen: Halogens are a group of chemically related elements, including fluorine, chlorine, bromine, iodine, astatine, and tennessine. Astatine and tennessine are radioactive. They are known as group 17 in the periodic table.

Fluorine: Fluorine (F) is a chemical element with atomic number 9. It is the lightest halogen, appearing as a toxic, pale yellow gas at standard conditions. Being the most electronegative element, fluorine is highly reactive and interacts with all elements except the light inert gases.

Chlorine: Chlorine (Cl) is a chemical element with atomic number 17. It is a yellow-green gas at room temperature and appears between fluorine and bromine in the periodic table. Chlorine is highly reactive and serves as a powerful oxidizing agent. It possesses the highest electron affinity and the third-highest electronegativity among the elements, only surpassed by oxygen and fluorine.

Bromine: Bromine is a chemical element with the symbol Br and atomic number 35. It is a red-brown liquid at room temperature that easily evaporates into a vapour of the same color. It possesses properties that are in between chlorine and iodine. The name "bromine" comes from the Ancient Greek word βρῶμος (bromos), meaning "stench," due to its strong and pungent smell. It was discovered independently by chemists Carl Jacob Löwig and Antoine Jérôme Balard.

Iodine: Iodine is a chemical element (symbol I, atomic number 53) and a stable halogen. It is a semi-lustrous, non-metallic solid that turns into a deep violet liquid at 114 °C and a violet gas at 184 °C. It was discovered in 1811 by Bernard Courtois and named by Joseph Louis Gay-Lussac after its violet color.

Astatine: Astatine (At) is a chemical element with atomic number 85. It is the rarest naturally occurring element and is only found as a decay product. All astatine isotopes are short-lived, with a half-life of 8.1 hours for the most stable isotope, astatine-210. Due to its radioactivity, a solid sample of astatine has never been observed as it instantly vaporizes.

Noble gas: The noble gases are elements in group 18 of the periodic table, including helium, neon, argon, krypton, xenon, and radon. They are odorless, colorless gases with low reactivity and extremely low boiling points under standard conditions.

Neon: Neon, symbol Ne, is an inert, colorless gas present in the periodic table as the second noble gas. With an atomic number of 10, it is odorless and monatomic, having approximately two-thirds the density of air under standard conditions.

Argon: Argon is a noble gas and the third most abundant gas in Earth's atmosphere at 0.934%. It is also the most abundant noble gas in the Earth's crust at 0.00015%.

Krypton: Krypton is a chemical element (Kr) with atomic number 36. It is a colorless, odorless, tasteless noble gas found in small quantities in the atmosphere. Krypton is frequently used in fluorescent lamps, along with other rare gases, and it is chemically unreactive.

Xenon: Xenon is a dense, colorless, odorless noble gas with symbol Xe and atomic number 54. It occurs in small quantities in Earth's atmosphere. Although unreactive, it can form xenon hexafluoroplatinate, the first synthesized noble gas compound.

Radon: Radon is an odorless and colorless radioactive noble gas. It has atomic number 86 and symbol Rn. Among the naturally occurring radon isotopes, only radon-222 has a long enough half-life to be released from soil and rock. Radon is constantly generated from the decay of uranium-238 and thorium-232, which have very long half-lives. This element has a short half-life of 3.8 days but will remain on Earth for billions of years. Radon produces other short-lived nuclides, referred to as "radon daughters," which eventually decay into stable isotopes of lead. Radon-222 is part of the uranium series, while radon-220 is part of the thorium series.

Transition metal: Transition metals are elements in the d-block of the periodic table, excluding group 12. Inner transition metals, including lanthanides and actinides, are also sometimes classified as transition metals.

Scandium: Scandium is a silvery-white metallic chemical element with symbol Sc and atomic number 21. It was discovered in 1879 by spectral analysis of minerals from Scandinavia. Although once considered a rare-earth element, it is now classified in the d-block.

Yttrium: Yttrium is a chemical element with symbol Y and atomic number 39. It is a silvery-metallic transition metal, similar to the lanthanides. Usually found in combination with lanthanides in rare-earth minerals, it is never found in nature as a free element. The only stable isotope, 89Y, is the sole isotope found in the Earth's crust.

Lutetium: Lutetium (Lu) is a silvery white metal that resists corrosion in dry air but not in moist air. It is the last element in the lanthanide series, classified as both a rare earth element and the first element of the 6th-period transition metals.

Titanium: Titanium (symbol Ti, atomic number 22) is a chemical element. It is a lustrous transition metal with a silver color, known for its low density and high strength. Resistant to corrosion in sea water, aqua regia, and chlorine, titanium is found in nature only as an oxide.

Zirconium: Zirconium is a chemical element with symbol Zr and atomic number 40. It is a lustrous, grey-white, strong transition metal. Derived from the mineral zircon, it is mainly used as a refractory and opacifier due to its resistance to corrosion. Zirconium forms various inorganic and organometallic compounds. It has five naturally occurring isotopes, four of which are stable. Zirconium compounds have no known biological role.

Hafnium: Hafnium is a chemical element with atomic number 72, symbol Hf, and is found in zirconium minerals. It is a silvery gray metal that resembles zirconium and was predicted by Dmitri Mendeleev in 1869 but discovered in 1922. Hafnium is one of the last two stable elements to be identified and is named after Hafnia, the Latin name for Copenhagen, where it was discovered.

Vanadium: Vanadium, symbol V, is a metallic element with atomic number 23. It is silvery-grey, hard, and malleable. Although rare in nature, it can be isolated artificially. The metal forms an oxide layer that protects it from further oxidation.

Niobium: Niobium is a light grey, crystalline transition metal with symbol Nb and atomic number 41. It is often used in jewelry as a hypoallergenic alternative to nickel due to its slow oxidation. Niobium is found in minerals like pyrochlore and columbite. Its name comes from Greek mythology and it has physical and chemical properties similar to tantalum.

Tantalum: Tantalum is a hard, corrosion-resistant metal with atomic number 73. It is named after Tantalus from Greek mythology and is part of the refractory metals group. It is often found with niobium in minerals like tantalite and coltan. Tantalum is used in high-melting-point alloys due to its strength and ductility.

Chromium: Chromium is a chemical element (Cr) with atomic number 24. It belongs to group 6 and is known for being a hard, brittle, lustrous, and steely-grey transition metal.

Molybdenum: Molybdenum, symbol Mo and atomic number 42, is a chemical element. Its name comes from Ancient Greek Μόλυβδος molybdos, meaning lead, due to its ores resembling lead ores. Discovered in 1778 by Carl Wilhelm Scheele and isolated in 1781 by Peter Jacob Hjelm, molybdenum minerals have a long history.

Tungsten: Tungsten is a rare chemical element (symbol W, atomic number 74), primarily found in compounds with other elements. Discovered in 1781, it became a metal in 1783. Important ores, such as scheelite and wolframite, contribute to its alternative name.

Manganese: Manganese, symbol Mn, is a chemical element with atomic number 25. It is a hard, brittle, and silvery metal found in minerals along with iron. First isolated in the 1770s, manganese serves various industrial alloy uses, especially in stainless steels, enhancing strength, workability, and wear resistance. Moreover, manganese compounds like oxide and sulfate have applications as an oxidizing agent, rubber additive, in glass making, fertilizers, ceramics, and as a fungicide.

Technetium: Technetium is a chemical element with symbol Tc and atomic number 43. It is the lightest radioactive element with all its isotopes being radioactive. It is mostly produced synthetically. It can be found naturally as a result of spontaneous fission in uranium and thorium ores, or as a result of neutron capture in molybdenum ores. It is a silvery gray transition metal located between manganese and rhenium in the periodic table, with properties intermediate to both elements. The most common naturally occurring isotope is 99Tc, but only in small amounts.

Rhenium: Rhenium (Re) is a rare, silvery-gray, heavy element with atomic number 75. It belongs to the transition metals in group 7 of the periodic table. Rhenium has the third-highest melting point and second-highest boiling point among all elements. It is primarily obtained as a by-product during the extraction and refinement of molybdenum and copper ores. With a concentration of only 1 part per billion in the Earth's crust, rhenium is one of the rarest elements. It exhibits a wide range of oxidation states in its compounds, from -1 to +7, and chemically resembles manganese and technetium.

Iron: Iron is a common chemical element (symbol Fe, atomic number 26) and a metal from the first transition series and group 8 on the periodic table. It is the most abundant element on Earth, found in the core and crust, often in its metallic form. Its ores can also be found in meteorites.

Ruthenium: Ruthenium is a rare metal belonging to the platinum group, discovered in 1844 and named after Russia. It is inert to most chemicals and usually found in small amounts in platinum ores. The annual production has increased from 19 to 35.5 tonnes. Ruthenium is used in electrical contacts, resistors, platinum alloys, and as a chemistry catalyst. It is also used as a capping layer for extreme ultraviolet photomasks. Ruthenium is found in the Ural Mountains, North and South America, Sudbury in Ontario, and South Africa.

Osmium: Osmium is a dense, bluish-white transition metal found in alloys, often in platinum ores. It is the densest naturally occurring element, with a density of 22.59 g/cm3. Osmium alloys are used in various applications requiring extreme durability and hardness, such as fountain pen nib tipping and electrical contacts.

Cobalt: Cobalt is a chemical element (Co) found in the Earth's crust in a chemically combined form. It is also present in alloys of natural meteoric iron. When obtained as a free element through reductive smelting, it is a hard, lustrous, and silvery metal.

Rhodium: Rhodium (Rh) is a highly rare and valuable chemical element. It belongs to the platinum group and is a silvery-white, hard, and corrosion-resistant transition metal. Rhodium is a noble metal, with only one naturally occurring isotope (103Rh). It mainly occurs as a free metal or alloy in minerals such as bowieite and rhodplumsite. Rhodium is one of the scarcest and most precious metals.

Iridium: Iridium is a dense, silvery-white transition metal with high corrosion resistance, belonging to the platinum group. It is the second-densest naturally occurring metal and can withstand temperatures up to 2,000°C. Though very resistant to corrosion, finely divided iridium dust can be flammable.

Nickel: Nickel is a silvery-white metal (symbol Ni, atomic number 28). It is hard and ductile, with a slight golden tinge. When exposed to air, it forms a protective layer of nickel oxide, which prevents corrosion. Nickel is rare in the Earth's crust and mostly found in ultramafic rocks and nickel-iron meteorites.

Palladium: Palladium is a rare chemical element with symbol Pd and atomic number 46. It was discovered in 1802 by William Hyde Wollaston and named after the asteroid Pallas. Palladium belongs to the platinum group metals (PGMs) and shares similar chemical properties with platinum, rhodium, ruthenium, iridium, and osmium. It has a lustrous silvery-white appearance and is known for having the lowest melting point and being the least dense among the PGMs.

Platinum: Platinum (Pt) is a dense and malleable chemical element, belonging to the silver-white transition metals. With atomic number 78, it is highly unreactive and precious. The name originated from the Spanish word "platina," meaning silver.

Copper: Copper (Cu) is a soft, malleable metal with high thermal and electrical conductivity. It is used as a conductor of heat and electricity, as a building material, and in various alloys like sterling silver, cupronickel, and constantan. It has a pinkish-orange color and is often utilized in jewelry, marine hardware, coins, and temperature measurement devices.

Silver: Silver is a chemical element with symbol Ag and atomic number 47. It is a soft, white, lustrous metal known for its high electrical and thermal conductivity, as well as reflectivity. Silver is found in its pure form, as well as in alloys with gold and other metals, and in minerals like argentite and chlorargyrite. It is mainly produced as a byproduct in the refining of copper, gold, lead, and zinc.

Gold: Gold is a dense and soft metal with atomic number 79. It is a transition metal, one of the noble metals, and has low reactivity.

Zinc: Zinc is a chemical element with symbol Zn and atomic number 30. It is a slightly brittle metal with a shiny-greyish appearance. Zinc shares chemical similarities with magnesium and has only one normal oxidation state (+2). It is the 24th most abundant element in the Earth's crust and is found in the zinc sulfide mineral sphalerite. Major zinc deposits are located in Australia, Asia, and the United States. The ore is refined through froth flotation, roasting, and electrowinning.

Cadmium: Cadmium is a soft, silvery-white metal with symbol Cd and atomic number 48. It is chemically similar to zinc and mercury, displaying oxidation state +2 in most compounds. Unlike transition metals, cadmium and its group 12 congeners lack partially filled d or f electron shells in their elemental forms. The average concentration of cadmium in Earth's crust ranges between 0.1 and 0.5 ppm. It was discovered in 1817 as an impurity in zinc carbonate by Stromeyer and Hermann in Germany.

Mercury (element): Mercury (Hg) is a heavy silvery d-block element with atomic number 80, symbol Hg. It is commonly known as quicksilver and was formerly called hydrargyrum, derived from Greek words meaning water and silver. Unlike other metallic elements, mercury remains liquid at standard temperature and pressure. The only other element that is liquid under these conditions is the halogen bromine.

Lanthanide: Lanthanides are a group of 14 metallic chemical elements (atomic numbers 57-70) that occupy the 4f orbitals in the periodic table. Although lutetium is a transition metal, it is sometimes included in the lanthanide series.

Lanthanum: Lanthanum (La) is a soft, silvery-white metal that slowly tarnishes when exposed to air. It belongs to the lanthanide series, a group of 15 similar elements, and is the first and prototype of this series. Lanthanum is a rare earth element with atomic number 57 and is commonly found in compounds with an oxidation state of +3. It has no biological role in humans but is essential to certain bacteria and exhibits mild antimicrobial activity.

Cerium: Cerium is a chemical element with symbol Ce and atomic number 58. It is a soft, ductile, silvery-white metal that tarnishes when exposed to air. It belongs to the lanthanide series and exhibits both a characteristic +3 oxidation state and a stable +4 state. While it is one of the rare-earth elements, it has no known biological role in humans and is not highly toxic, except with intense or prolonged exposure.

Praseodymium: Praseodymium is a rare-earth metal with atomic number 59. It is soft, silvery, and possesses magnetic, electrical, chemical, and optical properties. Found in the lanthanide series, it develops a green oxide coating when exposed to air due to its reactivity.

Neodymium: Neodymium (Nd) is a rare-earth metal with atomic number 60. It is a hard, silvery metal that tarnishes quickly in air. Neodymium has complex spectra and produces colorful compounds when oxidized. Discovered in 1885, it is commonly found in minerals like monazite and bastnäsite. Neodymium is refined for general use and is as common as cobalt, nickel, or copper in the Earth's crust. China dominates neodymium mining, similar to other rare-earth metals.

Promethium: Promethium is a rare, radioactive chemical element with symbol Pm and atomic number 61. It is one of only two radioactive elements followed by stable elements on the periodic table (the other being technetium). Promethium is a lanthanide and has only one stable oxidation state of +3. It is extremely scarce, with approximately 500-600 grams occurring naturally in the Earth's crust at any given time.

Samarium: Samarium is a chemical element with symbol Sm and atomic number 62. It is a moderately hard silvery metal that slowly oxidizes in air. It belongs to the lanthanide series and typically has an oxidation state of +3. Besides compounds with an oxidation state of +3, there are also compounds of samarium(II), including SmO, SmS, SmSe, SmTe, and SmI2.

Europium: Europium (Eu) is a silvery-white metal and a chemical element in the lanthanide series. It is highly reactive, soft, and less dense than other lanthanides. It can form a dark oxide coating when exposed to air. Europium was named after Europe and was first isolated in 1901. It usually has an oxidation state of +3, but compounds with +2 are common. While europium has no significant biological role, it is relatively non-toxic. Its main applications involve the phosphorescent properties of its compounds. Europium is one of the rarest elements in the rare-earth group.

Gadolinium: Gadolinium is a silvery-white metal, symbol Gd, with atomic number 64. It is malleable and ductile, reacting slowly with oxygen or moisture to form a black coating. Below 20°C, it is ferromagnetic, with a higher attraction to magnetic fields than nickel. Above this temperature, it is the most paramagnetic element. Gadolinium is found in nature in an oxidized form and often contains impurities of other rare-earths due to their similar chemical properties.

Terbium: Terbium (Tb) is a silvery-white, rare earth metal that belongs to the lanthanide series. Being malleable and ductile, it reacts with water, releasing hydrogen gas. Although never found freely in nature, terbium is present in various minerals like cerite, gadolinite, monazite, xenotime, and euxenite.

Dysprosium: Dysprosium is a rare-earth element with the symbol Dy and atomic number 66. It possesses a metallic silver luster and is commonly found in minerals like xenotime. Unlike other lanthanides, dysprosium does not exist freely in nature. It is composed of seven isotopes, with 164Dy being the most abundant.

Holmium: Holmium (Ho) is a rare-earth element with atomic number 67. It is a soft, silvery metal that is fairly corrosion-resistant and malleable. It forms a yellowish oxide coating when exposed to air but is stable in dry air. Holmium reacts with water, corrodes readily, and burns in air when heated.

Erbium: Erbium (Er) is a silvery-white rare-earth metal with atomic number 68. It is always found combined with other elements and was originally discovered in Ytterby, Sweden.

Thulium: Thulium (Tm) is a chemical element, with atomic number 69. It belongs to the lanthanide series. Thulium commonly exists in the +3 oxidation state, but can also be stable in the +2 oxidation state. Thulium compounds can form coordination complexes with nine water molecules in aqueous solutions.

Ytterbium: Ytterbium (Yb) is a chemical element with atomic number 70, belonging to the lanthanide series. It is a metal and its most common oxidation state is +3. Ytterbium compounds form complexes with nine water molecules in aqueous solution. It has lower density, melting point, and boiling point compared to other lanthanides due to its closed-shell electron configuration.

Actinide: The actinide series consists of 14 metallic elements (atomic numbers 89 to 102), starting with actinium. The chemical symbol An is commonly used to refer to any actinide in discussions on actinide chemistry.

Actinium: Actinium is a chemical element with the symbol Ac and atomic number 89. It was first isolated in 1902 and wrongly identified with a substance called actinium. Actinium gave its name to the actinide series, a group of 15 elements. Alongside polonium, radium, and radon, actinium is one of the first non-primordial radioactive elements to be isolated.

Thorium: Thorium (Th) is a weakly radioactive metal with atomic number 90. It tarnishes to an olive gray color when exposed to air, forming thorium dioxide. It is moderately soft and malleable with a high melting point. Thorium is an electropositive actinide that primarily exists in the +4 oxidation state. It is highly reactive and can combust in air when finely divided.

Protactinium: Protactinium (Pa) is a dense, silvery-gray metal that readily reacts with oxygen, water vapor, and acids. It mainly exists in the +5 oxidation state but can also be found in +4, +3, or +2 states. It is highly radioactive and toxic. Found in very low concentrations in the Earth's crust, it can reach slightly higher levels in some uraninite ore deposits. Due to its scarcity and hazards, protactinium is only used in scientific research, primarily extracted from spent nuclear fuel.

Uranium: Uranium is a chemical element with atomic number 92. It is a silvery-grey metal in the periodic table's actinide series. Uranium undergoes radioactive decay and has a long half-life, making it useful for dating the age of the Earth. The most abundant isotopes are uranium-238 and uranium-235. It is dense and occurs naturally in low concentrations in soil, rock, and water, being extracted from minerals like uraninite for commercial use.

Neptunium: Neptunium (Np) is a radioactive actinide metal with atomic number 93. It is the first transuranic element and named after Neptune due to its position in the periodic table. Neptunium metal tarnishes and occurs in three allotropic forms. It exhibits oxidation states from +3 to +7. Being radioactive and poisonous, neptunium is dangerous to handle as it can accumulate in bones.

Plutonium: Plutonium (Pu) is a radioactive chemical element with atomic number 94. It is a silvery-gray metal that tarnishes and forms a dull coating when exposed to air. Plutonium has six allotropes, four oxidation states, and reacts with various elements. When in contact with moist air, it forms oxides and hydrides, resulting in a volume expansion of up to 70% and the release of a pyrophoric powder. Due to its accumulation in bones, plutonium is highly dangerous to handle.

Americium: Americium is a synthetic radioactive element with symbol Am and atomic number 95. It is a transuranic member of the actinide series, found below europium on the periodic table. Named after the Americas, it possesses these distinguishing characteristics.

Curium: Curium (Cm) is a synthetic chemical element with atomic number 96. It is a transuranic actinide named after Marie and Pierre Curie, renowned for their research on radioactivity. In 1944, curium was intentionally created by bombarding plutonium with alpha particles. Its discovery was kept secret until after World War II and was announced to the public in 1947. Curium is primarily produced in nuclear reactors by bombarding uranium or plutonium with neutrons. A tonne of spent nuclear fuel contains around 20 grams of curium.

Berkelium: Berkelium (Bk) is a synthetic element with atomic number 97. It belongs to the actinide and transuranium series. Discovered in December 1949 at the Lawrence Berkeley National Laboratory in Berkeley, California, it is named after the city. Berkelium was the fifth transuranium element found, following neptunium, plutonium, curium, and americium.

Californium: Californium (Cf) is a synthetic chemical element with atomic number 98. It was first created in 1950 by bombarding curium with alpha particles at Lawrence Berkeley National Laboratory. Californium is an actinide element, the sixth transuranium element synthesized, and has a high atomic mass. It is named after the state of California and was one of the few elements produced in quantities visible to the naked eye.

Synthetic element: A synthetic element is a man-made chemical element not naturally found on Earth. These elements, with atomic numbers 95-118, were created by manipulating particles in nuclear reactors, particle accelerators, or atomic bomb explosions. Synthetic elements are unstable and decay at different rates, with half-lives ranging from microseconds to millions of years.

Acid: An acid is a molecule or ion that can donate a hydrogen ion (H+) or form a covalent bond with an electron pair. It acts as a Brønsted–Lowry acid or a Lewis acid, respectively.

Base (chemistry): "Base (chemistry)" refers to substances that react with acids and has three common definitions: Arrhenius bases, Brønsted bases, and Lewis bases. This term was first proposed by G.-F. Rouelle in the mid-18th century.

pH: pH is a measure of acidity or basicity in chemistry, symbolized with "potential of hydrogen". It defines the acidity or basicity of water-based solutions, with lower pH values indicating acidity and higher values indicating alkalinity.

Boric acid: Boric acid, also known as orthoboric acid, is a compound containing boron, oxygen, and hydrogen. It is a weak acid that can dissolve in water and is found in nature as the mineral sassolite. It appears as colorless crystals or a white powder. Boric acid can form different borate anions and salts, and can react with alcohols to create borate esters.

Hydrochloric acid: Hydrochloric acid, also called muriatic acid, is a strong, colorless solution of hydrogen chloride (HCl). It has a pungent smell and is commonly found in the digestive system of animals, including humans. Apart from being a laboratory reagent and industrial chemical, it plays a vital role as a component of gastric acid.

Hydrofluoric acid: Hydrofluoric acid (HF) is a colorless and highly corrosive solution of hydrogen fluoride in water. It is utilized in the production of fluorine-containing compounds, including popular medications like fluoxetine (Prozac) and materials like Teflon. It is also used for glass and silicon wafer etching and serves as a source for elemental fluorine.

Nitric acid: Nitric acid (HNO3) is a highly corrosive mineral compound that is colorless in its pure form. It can turn yellow over time due to decomposition into nitrogen oxides. Commercially available nitric acid is usually 68% concentration in water. When the concentration exceeds 86%, it is called fuming nitric acid, further classified as red fuming nitric acid (above 86%) or white fuming nitric acid (above 95%) based on nitrogen dioxide content.

Phosphoric acid: Phosphoric acid (H3PO4) is an odorless and colorless inorganic compound, commonly found in an 85% aqueous solution. It is widely used in industrial applications and is a crucial ingredient in various fertilizers.

Sulfuric acid: Sulfuric acid, also known as oil of vitriol, is a mineral acid made up of sulfur, oxygen, and hydrogen (H2SO4). It is a colorless, odorless, and thick liquid that can mix with water.

Carboxylic acid: A carboxylic acid is an organic acid that contains a carboxyl group attached to an R-group. Its general formula is written as R-COOH or R-CO2H, with R representing different groups. Carboxylic acids are widely found in nature, including amino acids and fatty acids. When a carboxylic acid loses a proton, it forms a carboxylate anion.

Acetic acid: Acetic acid, also known as ethanoic acid, is a colorless liquid that is the main component of vinegar. It has a chemical formula of CH3COOH and has been used in vinegar for centuries. Vinegar contains at least 4% acetic acid besides water.

Citric acid: Citric acid is a weak organic acid found in citrus fruits, playing a central role in the metabolism of aerobic organisms as an intermediate in the citric acid cycle.

Lactic acid: Lactic acid is an organic acid with the molecular formula CH3CH(OH)COOH. It is miscible with water and exists as a white solid or colorless solution. It can be produced synthetically or found naturally. Lactic acid is an alpha-hydroxy acid and is widely used in organic synthesis and biochemical industries. Its conjugate base is called lactate, and the derived acyl group is lactoyl.

Alloy: An alloy is a mixture of chemical elements, with at least one being a metal. It retains the properties of a metal, such as electrical conductivity and luster, but can also have different properties like increased strength. Alloys can reduce material costs while maintaining important characteristics, and they can also enhance properties like corrosion resistance or mechanical strength.

Amalgam (chemistry): Amalgam is a metal alloy containing mercury that can be a liquid, paste, or solid. It is formed through metallic bonding, binding metal ions together. Most metals can form amalgams except for iron, platinum, tungsten, and tantalum. Silver-mercury amalgams are used in dentistry, while gold-mercury amalgam is used to extract gold from ore. Dentistry also uses mercury alloys with metals like silver, copper, indium, tin, and zinc.

Brass: Brass is a versatile alloy of copper and zinc, with diverse properties and color variations. With copper typically having a higher proportion, it has been used since ancient times. It is a substitutional alloy allowing atoms of copper and zinc to replace each other within the same crystal structure.

Bronze: Bronze is a copper-based alloy commonly made with around 12-12.5% tin and other metals. It can also contain non-metals like phosphorus or metalloids such as arsenic or silicon. These additions create a variety of alloys with helpful properties like hardness, strength, ductility, and machinability.

Cast iron: Cast iron is a type of iron made with more than 2% carbon and around 1-3% silicon. It has a low melting temperature, making it useful for various applications. Different types of cast iron have different formations of carbon. White cast iron has hard and brittle carbon called cementite, while grey cast iron has graphite flakes that deflect cracks and initiate new ones. Ductile cast iron has spherical graphite nodules that prevent cracks from spreading.

Steel: Steel is a strong and resilient alloy made of iron and carbon. It possesses enhanced strength and fracture resistance compared to other forms of iron. Steel can be further improved by adding various elements. Stainless steel, for example, requires 11% chromium to resist corrosion and oxidation. Due to its low cost and high tensile strength, steel finds utility in buildings, infrastructure, tools, transportation, machinery, appliances, furniture, and weaponry.

Stainless steel: Stainless steel, also called inox, is a corrosion-resistant alloy of iron. It contains chromium (at least 10.5%) and often nickel, with carbon ranging from 0.2 to 2.11%. The chromium forms a protective film that defends against corrosion and allows self-healing in the presence of oxygen.

Wrought iron: Wrought iron is a low carbon iron alloy, distinguished from cast iron. It has a fibrous slag structure that resembles wood grain when etched, rusted, or bent. Wrought iron is tough, malleable, ductile, and corrosion resistant. It can be easily forge welded but is less suitable for electrical welding.

Syngas: Syngas is a mixture of hydrogen and carbon monoxide, often with some carbon dioxide and methane. It is used primarily for producing ammonia or methanol. Syngas is combustible and can be used as a fuel. It has been historically used as a gasoline replacement, such as wood gas during WWII to power cars in Europe.

Carbon monoxide: Carbon monoxide is a toxic, flammable gas that is invisible and odorless. It is formed by a bond between a carbon and an oxygen atom. Commonly known as CO, it is found in industrial processes and its presence can be harmful.

Carbon dioxide: Carbon dioxide (CO2) is a chemical compound that consists of one carbon atom bonded to two oxygen atoms. It is a gas at room temperature and serves as the primary carbon source for life on Earth through the carbon cycle. Carbon dioxide is transparent to visible light but absorbs infrared radiation, making it a greenhouse gas. It is soluble in water and found in various sources such as groundwater, lakes, ice caps, and seawater. When dissolved in water, it forms carbonate and bicarbonate, contributing to ocean acidification.

Nitrous oxide: Nitrous oxide, or laughing gas, is a colourless non-flammable gas with a slightly sweet smell. It is an oxide of nitrogen and acts as a strong oxidizer at high temperatures, resembling molecular oxygen.

Ammonia: Ammonia (NH3) is a colourless gas with a strong smell. It serves as a nitrogenous waste and is used in fertilisers. 70% of industrially produced ammonia is used for fertiliser production, including urea and diammonium phosphate. Additionally, pure ammonia is directly applied to soil.

Hydrogen peroxide: Hydrogen peroxide (H2O2) is a pale blue liquid used as an oxidizer, bleaching agent, and antiseptic. It is slightly more viscous than water and commonly found in diluted form for consumer use. In higher concentrations, it is used for industrial purposes. Concentrated hydrogen peroxide, known as "high-test peroxide", can explode when heated and has been utilized in rocketry as both a monopropellant and an oxidizer.

Water: Water (H2O) is an essential and abundant inorganic compound found on Earth. It is a colorless, tasteless, and odorless substance that makes up a significant part of our planet's hydrosphere and all living organisms. With a unique chemical structure, each water molecule contains one oxygen atom bonded to two hydrogen atoms. Despite not providing food energy or organic nutrients, water is vital for all forms of life. At standard conditions, "water" refers to its liquid state.

Properties of water: Water is a polar compound that is tasteless, odorless, and nearly colorless. It is known as the "universal solvent" and "solvent of life" due to its ability to dissolve many substances. Water is the most abundant substance on Earth's surface and can exist as a solid, liquid, and gas. It is also the third most abundant molecule in the universe.

Ice: Ice is solid water formed at or below freezing temperatures. It has an ordered structure and is considered a mineral. It can be transparent or bluish-white depending on impurities.

Water vapor: Water vapor is the gaseous phase of water found in the hydrosphere. It can be produced through evaporation, boiling, or ice sublimation. Being transparent, it contributes to the atmosphere and is continuously generated by evaporation and removed by condensation. Water vapor's low density triggers convection currents, which can result in the formation of clouds and fog.

Organophosphorus chemistry: Organophosphorus chemistry is the study of phosphorus-containing organic compounds. These compounds are used in pest control as alternatives to environmentally persistent chlorinated hydrocarbons. Some organophosphorus compounds are powerful insecticides but others, such as sarin and VX nerve agents, are highly toxic to humans.

Organosilicon chemistry: Organosilicon chemistry involves the study of compounds that have carbon-silicon bonds. These compounds, known as organosilicon compounds, are similar to organic compounds in many ways. They are colorless, flammable, hydrophobic, and stable to air. However, silicon carbide is an exception to this as it is an inorganic compound.

Silicone: Silicone is a versatile polymer made of siloxane units. It can exist as colorless oils or rubber-like materials. Silicone finds wide applications in sealants, adhesives, lubricants, medicine, cooking utensils, thermal insulation, and electrical insulation. Various forms of silicone include oil, grease, rubber, resin, and caulk.

Organosulfur chemistry: Organosulfur chemistry is the study of sulfur-containing organic compounds. These compounds can have foul odors or be sweet, like saccharin. Sulfur is vital for life and is found in important substances such as antibiotics and amino acids. Fossil fuels also contain organosulfur compounds, which oil refineries work to remove.

Thiol: A thiol is an organosulfur compound with the formula R−SH. It contains the −SH functional group called thiol or sulfhydryl or sulfanyl group. Thiols are sulfur versions of alcohols, combining "thio-" and "alcohol."

Hydrocarbon: Hydrocarbons are organic compounds made up of hydrogen and carbon. They are colorless, hydrophobic, and can have a faint odor similar to gasoline or lighter fluid. Hydrocarbons can exist as gases, liquids, low melting solids, or polymers, with a wide variety of molecular structures.

Petroleum: Petroleum, also called crude oil or simply oil, is a natural liquid composed mostly of hydrocarbons. It is found in geological formations and includes both unprocessed crude oil and refined petroleum products.

Alkane: An alkane, also known as a paraffin, is a type of organic compound made up of hydrogen and carbon atoms arranged in a tree-like structure. All carbon-carbon bonds in alkane molecules are single. Alkanes have the formula CnH2n+2, where n represents the number of carbon atoms. They can range from simple, like methane, to highly complex molecules such as pentacontane or 6-ethyl-2-methyl-5-(1-methylethyl) octane.

Methane: Methane (CH4) is the simplest alkane and a major component of natural gas. Its abundance on Earth makes it a valuable fuel, but capturing and storing it is challenging due to its gaseous state under normal conditions.

Ethane: Ethane is a natural and odorless gas with the chemical formula C2H6. It is obtained from natural gas and petroleum refining. Ethane is primarily used as a raw material for producing ethylene.

Propane: Propane is a three-carbon alkane gas commonly used as a fuel in domestic and industrial applications. It is compressible to a transportable liquid and burns more cleanly than gasoline and coal. Discovered in 1857, it became commercially available in the US by 1911. Propane is part of a group of liquefied petroleum gases and is a by-product of natural gas processing and petroleum refining.

Butane: Butane is a flammable, colorless gas with the formula C4H10 that easily evaporates at room temperature. The name originates from butyric acid, meaning butter in Greek. It was first discovered in crude petroleum in 1864 and commercially used in the early 1910s by Walter O. Snelling.

Alkene: An alkene, or olefin, is an organic compound containing a carbon–carbon double bond, which can be either internal or at the terminal position. Terminal alkenes are also referred to as α-olefins.

Ethylene: Ethylene, C2H4, is a colorless, flammable gas with a faint "sweet and musky" odor. It is the simplest alkene and serves as an important hydrocarbon in various industries.

Propylene: Propylene, or propene, is an unsaturated organic compound (CH3CH=CH2) belonging to the alkene class of hydrocarbons. It has one double bond, appears as a colorless gas, and has a subtle petroleum-like smell.

Alkyne: An alkyne is a type of unsaturated hydrocarbon with at least one carbon-carbon triple bond. It belongs to a homologous series with the formula CnH2n−2. Alkynes are also called acetylenes, with C2H2 known as ethyne. They are generally hydrophobic, like other hydrocarbons.

Aromatic compound: Aromatic compounds, also known as arenes, are organic compounds with a unique chemistry characterized by cyclic conjugation, notably exemplified by benzene. The term "aromatic" originated from odor-based categorization, but it now refers to compounds that satisfy Hückel's Rule and have specific properties. These compounds are typically unreactive, nonpolar, and hydrophobic, with a high carbon-hydrogen ratio. They burn with a distinctive yellow flame and undergo electrophilic and nucleophilic substitutions.

Benzene: Benzene is an organic chemical compound (C6H6) composed of a hexagonal ring with six carbon atoms and a hydrogen atom attached to each. It is classified as a hydrocarbon due to the presence of only carbon and hydrogen atoms.

Alcohol (chemistry): An alcohol is an organic compound with a hydroxyl group attached to a saturated carbon atom. Alcohols can be simple, like methanol and ethanol, or complex, like sucrose and cholesterol. The hydroxyl group gives alcohols hydrophilic properties and provides a site for various reactions.

Methanol: Methanol is a basic alcohol compound (CH3OH) that is colorless, flammable, and has a distinctive odor. It was commonly known as wood alcohol due to its historical production method from wood distillation. However, it is now predominantly produced by hydrogenation of carbon monoxide in industrial settings.

Ethanol: Ethanol, also known as ethyl alcohol, is a volatile and flammable organic compound with the formula CH3CH2OH. It has a characteristic wine-like smell and pungent taste. Ethanol works as a psychoactive recreational drug and is found as the main component in alcoholic beverages.

Phenol: Phenol, or Benzenol, is an aromatic organic compound (C6H5OH) that is a white volatile solid. It contains a phenyl group bonded to a hydroxy group and is mildly acidic. Careful handling is necessary due to its potential to cause chemical burns.

Aldehyde: An aldehyde is an organic compound with the R-CH=O functional group, also known as a formyl group. Aldehydes are widely found in technology and biology due to their presence in many important chemicals.

Formaldehyde: Formaldehyde is a pungent, colorless gas with the formula CH2O. It spontaneously forms paraformaldehyde and is stored as formalin. It is the simplest aldehyde and widely used in industry for manufacturing resins and coatings. Its global production rate in 2006 was 12 million tons per year. Small amounts of formaldehyde can also be found naturally.

Ketone: A ketone is an organic compound with a carbonyl group (−C=O) and can have various carbon-containing substituents. Acetone is the simplest ketone and has the formula (CH3)2CO. Ketones are significant in biology and industry, including in sugars, steroids, and the solvent acetone.

Acetone: Acetone (CH3)2CO is the smallest ketone, a colorless and flammable liquid with a pungent odor. It is highly volatile and an organic compound.

Amide: An amide, or organic amide, is a compound with the formula R−C(=O)−NR′R″, where R, R', and R″ represent various groups or hydrogen atoms. In proteins, the amide group is known as a peptide bond in the main chain or an isopeptide bond in the side chain. It is a derivative of a carboxylic acid with the hydroxyl group replaced by an amine group.

Urea: Urea, or carbamide, is an organic compound (CO(NH2)2) with two amino groups linked by a carbonyl group. It is the simplest amide of carbamic acid.

Nylon: Nylon is a synthetic polymer composed of amide backbones that link aliphatic or semi-aromatic groups.

Amine: Amines are compounds with a basic nitrogen atom and a lone pair. They are derivatives of ammonia, with hydrogen atoms replaced by substituents like alkyl or aryl groups. Key examples include amino acids, biogenic amines, trimethylamine, and aniline. Inorganic derivatives like monochloramine are also called amines.

Ester: An ester is a compound formed from an acid where the hydrogen atom in the acidic hydroxyl group is replaced by an organyl group. Ester category also includes compounds where oxygen is replaced by other chalcogens. Some authors consider derivatives of other acids as esters, but not according to IUPAC.

Ether: Ether is a type of compound in organic chemistry characterized by an oxygen atom bonded to two alkyl or aryl groups. It can be divided into simple (symmetrical) ethers with identical groups on either side of the oxygen atom, and mixed (unsymmetrical) ethers with different groups. Diethyl ether is an example of a simple ether widely used as a solvent and anesthetic. Ethers are important in both organic and biochemistry, often found in carbohydrates and lignin.

Haloalkane: Haloalkanes, also known as alkyl halides, are a type of compound that contain halogen substituents. They are commonly used for various purposes such as flame retardants, solvents, and pharmaceuticals, but they can also be harmful pollutants. Chlorine, bromine, and iodine-containing haloalkanes pose a threat to the ozone layer, while fluorinated haloalkanes may act as greenhouse gases. Methyl iodide, however, does not deplete the ozone layer. These compounds have the general formula "RX," where R is an alkyl or substituted alkyl group and X is a halogen.

Polymer: A polymer is a substance with large molecules called macromolecules, made up of many repeating subunits. Both natural and synthetic polymers have diverse properties that are essential in everyday life. They range from plastics like polystyrene to biopolymers like DNA and proteins. Polymers are produced by combining small molecules called monomers through a process called polymerization. Their large molecular mass gives them unique physical properties such as toughness, elasticity, and the ability to form amorphous or semicrystalline structures.

Polyethylene: Polyethylene (PE) is a widely used polymer and the most common plastic. It is mainly employed for packaging purposes like bags, films, and containers. With an annual production of over 100 million tonnes, it constitutes 34% of the total plastics market as of 2017.

Polypropylene: Polypropylene (PP), or polypropene, is a versatile thermoplastic polymer produced from propylene monomers through chain-growth polymerization. It finds widespread applications in various industries.

Polyvinyl chloride: Polyvinyl chloride (PVC) is a commonly used synthetic polymer that ranks as the third most widely produced plastic. It is produced in massive quantities, with around 40 million tons manufactured annually.

Tannin: Tannins are bitter, astringent substances that are found in various plants. They have the unique ability to bind and cause proteins, amino acids, and alkaloids to separate and form sediments.

Salt (chemistry): A salt in chemistry is a compound made of ions with opposite charges that balance each other, resulting in no net electric charge. For example, table salt consists of sodium ions carrying positive charges and chloride ions carrying negative charges.

Cyanide: Cyanide is a chemical compound with a C≡N functional group, consisting of a carbon atom triple-bonded to a nitrogen atom.

Chloride: Chloride refers to a negatively charged chlorine ion or a non-charged chlorine atom bonded to other molecules. It includes inorganic salts and organic compounds. The term 'chloride' is pronounced differently.

Calcium chloride: Calcium chloride (CaCl2) is an inorganic salt that is white and crystalline. It readily dissolves in water and is formed by reacting hydrochloric acid with calcium hydroxide.

Sodium chloride: Sodium chloride, also known as table salt, is an ionic compound (NaCl) consisting of sodium and chloride ions. It is responsible for seawater's salinity and the extracellular fluid in organisms. Besides being a condiment and food preservative, it is widely used in industries and serves as a source for sodium and chlorine compounds used in chemical syntheses. Sodium chloride is also applied for deicing roadways in sub-freezing weather.

Oxide: An oxide is a compound with at least one oxygen atom and another element in its formula. It is the dianion of oxygen, with oxygen in the oxidation state of −2. Oxides make up a significant part of the Earth's crust and even pure elements can develop oxide coatings, like aluminium foil with a protective Al2O3 layer.

Titanium dioxide: Titanium dioxide, also known as titanium(IV) oxide or titania, is an inorganic compound with the chemical formula TiO2. It is a white solid widely used as a pigment called titanium white in applications such as paint, sunscreen, and food coloring. It is insoluble in water but can appear black in mineral forms. With global production exceeding 9 million tonnes in 2014, titanium dioxide is estimated to be used in two-thirds of all pigments and valued at $13.2 billion. It is assigned E number E171 when used as a food coloring.

Silicate: Silicates are polyatomic anions consisting of silicon and oxygen with various formulas such as SiO4−4, SiO2−3, and Si2O6−7. They can form salts like sodium metasilicate and esters like tetramethyl orthosilicate. Silicates are also used to refer to anions containing silicon, sometimes with other atoms like fluorine. Silicates are commonly found as silicate minerals.

Sodium silicate: Sodium silicate is a group of chemical compounds with the formula Na2xSiyO2y+x, such as sodium metasilicate and sodium orthosilicate. These compounds are usually colorless solids or white powders that dissolve in water. Sodium silicate is widely used in various applications.

Sodium bicarbonate: Sodium bicarbonate, also known as baking soda or bicarbonate of soda, is a white solid commonly used in baking. It is composed of sodium and bicarbonate ions and has a slightly salty, alkaline taste. Sodium bicarbonate is often in a fine powder form and is found in mineral springs.

Carbonate: Carbonate is a salt of carbonic acid (H2CO3). It contains the carbonate ion (CO2−3). The term also refers to an organic compound with the carbonate group O=C(−O−)2.

Calcium carbonate: Calcium carbonate (CaCO3) is a common compound found in rocks like calcite and aragonite, as well as in chalk, limestone, shells, and pearls. It is considered calcareous. This substance is used as an active ingredient in agricultural lime and can be formed as limescale by the reaction of calcium ions and carbonate ions in hard water. Calcium carbonate has medical applications as a calcium supplement and antacid. However, excessive consumption can lead to health problems such as hypercalcemia and digestive issues.

Sodium carbonate: Sodium carbonate (Na2CO3) is an inorganic compound and its hydrates are white, odourless, water-soluble salts. It is known as "soda ash" and was historically extracted from sodium-rich plant ashes. Today, it is produced in large quantities through the Solvay process from sodium chloride and limestone, or by carbonating sodium hydroxide made using the Chlor-alkali process.

Potassium hydroxide: Potassium hydroxide (KOH) is an inorganic compound, commonly known as caustic potash. It has the chemical formula KOH and is highly corrosive.

Sodium hydroxide: Sodium hydroxide, or lye/caustic soda, is a white solid compound (NaOH) consisting of sodium cations (Na+) and hydroxide anions (OH−).

Nitrate: Nitrate (NO−3) is a polyatomic ion found in salts known as nitrates. It is commonly used in fertilizers and explosives. Most inorganic nitrates easily dissolve in water, but bismuth oxynitrate is an exception as it is insoluble.

Ammonium nitrate: Ammonium nitrate (NH4NO3) is a white crystalline salt used mainly as a water-soluble and hygroscopic fertilizer in agriculture. The compound consists of ammonium and nitrate ions, and it does not form hydrates.

Potassium nitrate: Potassium nitrate is a compound with the formula KNO3, and it has a sharp, salty, bitter taste. It is an ionic salt consisting of potassium ions (K+) and nitrate ions (NO3−). Found naturally as a mineral called niter, it is an alkali metal nitrate. It is used as a source of nitrogen and is a component of saltpeter, a group of nitrogen-containing compounds. Nitrogen was named after niter.

Sodium nitrate: Sodium nitrate (NaNO3) is a nitrate salt commonly referred to as Chile saltpeter. It is distinct from potassium nitrate and is also known as nitratine or soda niter in its mineral form.

Sulfate: Sulfate (SO2−4) is a common polyatomic anion found in salts, acid derivatives, and peroxides used across various industries. It is also present in everyday life. Sulfates are formed from sulfuric acid and are commonly used in their salt form.

Alum: Alum is a chemical compound consisting of hydrated double sulfate salts of aluminum. The general formula is XAl(SO4)2·12H2O, with X being a monovalent cation like potassium or ammonium. The term "alum" commonly refers to potassium alum, KAl(SO4)2·12H2O. Different alums take their names from the monovalent ion they contain, like sodium alum and ammonium alum.

Ammonium sulfate: Ammonium sulfate, also known as ammonium sulphate, is an inorganic salt used mainly as a soil fertilizer. It consists of 21% nitrogen and 24% sulfur, making it beneficial for plant growth.

Magnesium sulfate: Magnesium sulfate (MgSO4) is a white crystalline salt compound, soluble in water but not in ethanol. It contains magnesium cations (Mg2+) and sulfate anions (SO2−4), making up about 20.19% of its mass.

Sodium sulfate: Sodium sulfate, also called sodium sulphate, is an inorganic compound (Na2SO4) found in different hydrated forms. It is a white solid that easily dissolves in water. The decahydrate is widely produced and used as a filler in laundry detergents and in paper pulping processes, where it helps create alkaline sulfides.

Mass spectrometry: Mass spectrometry (MS) is an analytical technique measuring ion mass-to-charge ratio. It generates a mass spectrum plot showing intensity versus mass-to-charge ratio. Widely used, MS serves various fields and analyzes both pure samples and complex mixtures.

Spectroscopy: Spectroscopy is the study of electromagnetic spectra, specifically measuring and interpreting them. It encompasses the analysis of color across the entire electromagnetic spectrum, going beyond visible light.

Absorption spectroscopy: Absorption spectroscopy measures how electromagnetic radiation is absorbed by a sample, creating an absorption spectrum that shows how intensity changes with frequency or wavelength. This technique is used across the electromagnetic spectrum to understand the interaction between radiation and samples.

Radical (chemistry): A radical, or free radical, in chemistry refers to an entity with at least one unpaired valence electron. These unpaired electrons make radicals highly reactive, and they often combine with other radicals. Most organic radicals have short lifetimes.

Reaction mechanism: A reaction mechanism is the sequence of elementary reactions that lead to an overall chemical reaction. It provides a step-by-step understanding of how the reaction occurs.

Substitution reaction: A substitution reaction is an important type of chemical reaction in organic chemistry where one functional group is replaced by another. Substitution reactions can be electrophilic or nucleophilic based on the reagent and the reactive intermediate involved. Understanding these reactions helps predict products and optimize conditions for the reaction.

Redox: Redox refers to a chemical reaction where the oxidation states of a substance change. Oxidation involves losing electrons or increasing the oxidation state, while reduction involves gaining electrons or decreasing the oxidation state.

Corrosion: Corrosion is the natural deterioration of metals through chemical reactions, converting them into stable oxides. It occurs gradually due to interactions with the environment. Corrosion engineering aims to control and prevent this process.

Acid–base reaction: An acid-base reaction is a chemical reaction between an acid and a base. It can be used to determine pH through titration. Various acid-base theories, such as Brønsted-Lowry theory, offer alternative perspectives on the reaction mechanisms.

Electrolysis: Electrolysis is a process that utilizes electric current to facilitate non-spontaneous chemical reactions in manufacturing and chemistry. It plays a vital role in separating elements from natural sources like ores with the help of an electrolytic cell. The required voltage for electrolysis, known as decomposition potential, initiates the breakdown of substances through electricity. Overall, electrolysis can be defined as the electrically induced breakdown or separation of compounds.

Combustion: Combustion is a chemical reaction between a fuel and oxygen that produces gaseous products, often seen as smoke. It may or may not result in a visible flame, which indicates the reaction. Activation energy is needed to start combustion, but a flame can sustain the reaction with its heat.

Haber process: The Haber process, also known as the Haber-Bosch process, is a significant industrial method for producing ammonia. Developed by German chemists Fritz Haber and Carl Bosch in the early 1900s, this process converts atmospheric nitrogen to ammonia through a reaction with hydrogen using an iron catalyst under high temperatures and pressures. The reaction is slightly exothermic, favoring lower temperatures and higher pressures. Hydrogen is generated via steam reforming, and an iterative closed cycle is used to react hydrogen with nitrogen to create ammonia.

Chromatography: Chromatography is a lab technique used to separate mixtures into individual components. A fluid solvent known as the mobile phase carries the mixture through a system containing a fixed material called the stationary phase. The components of the mixture interact differently with the stationary phase, causing them to separate based on their affinities. The separation relies on the differential partitioning between the mobile and stationary phases, with compounds having different retention times.

Distillation: Distillation is a process that separates substances from a liquid mixture by boiling and condensing. It can either completely separate components or increase the concentration of selected ones. This physical separation method is widely used in various industries, such as in the production of distilled beverages, desalination, oil stabilization, and chemical synthesis. Distillation is also utilized to separate air into its components for industrial purposes.

Filtration: Filtration is a physical separation process using a complex filter to separate solid matter from fluid in a mixture. The fluid passes through, called the filtrate, while solid particles are described as oversize. There may be blinding or the formation of a filter cake. The filter's effective pore size determines the largest particles it can pass. Filtration is imperfect, contaminating solids with fluid and producing fine particles in the filtrate. It occurs in nature and engineered systems, including biological, geological, and industrial forms.

Valence bond theory: Valence bond theory is a chemical bonding theory that uses quantum mechanics to explain how atomic orbitals of dissociated atoms combine to form individual chemical bonds in a molecule. This theory contrasts with molecular orbital theory, which considers orbitals that span the entire molecule.

Chemical kinetics: Chemical kinetics, or reaction kinetics, is a branch of physical chemistry that studies the speed of chemical reactions. It explores the influence of experimental conditions on reaction rates and provides insights into reaction mechanisms and transition states. It differs from chemical thermodynamics, which focuses on reaction direction rather than rate. Chemical kinetics also involves developing mathematical models to describe the characteristics of reactions.

Intermolecular force: Intermolecular forces are the forces that allow molecules to interact with each other, including electromagnetic forces of attraction or repulsion. These forces are weaker than the forces that hold molecules together, such as covalent bonds. Intermolecular forces play a vital role in molecular mechanics models.

Earth science: Earth science, also known as geoscience, studies the Earth's complex constitution of its four spheres: biosphere, hydrosphere/cryosphere, atmosphere, and geosphere. It includes the physical, chemical, and biological aspects of the planet. Earth science is a branch of planetary science with a long history.

History of Earth: The Earth's history spans from formation to present day, with constant geological changes and biological evolution. Contributions from various branches of natural science have helped to understand significant events.

Extinction event: An extinction event refers to a widespread and rapid decline in Earth's biodiversity, characterized by a sharp decrease in the abundance and variety of multicellular organisms. It happens when the rate of extinction surpasses the background extinction rate and the rate of new species formation. There are varying estimates, ranging from as few as five to more than twenty, regarding the number of major mass extinctions in the past 540 million years. Disagreements arise from different interpretations of what defines a "major" extinction event and the selected data used to assess historical biodiversity.

Ice age: An ice age is a prolonged period of cold temperatures on Earth's surface and atmosphere, leading to the growth of ice sheets and glaciers. These ice ages alternate with warmer periods, known as interglacials, when glaciers recede. The current ice age is called Quaternary glaciation. This cycle includes glacial periods, characterized by colder climates, and interglacials, marked by intermittent warmer climates.

Natural disaster: A natural disaster is a highly harmful event caused by natural hazards such as floods, earthquakes, cyclones, and wildfires. It can cause loss of life, property damage, and economic devastation. Scholars suggest using the term "disaster" instead, while also specifying the type of hazard. A disaster occurs when a vulnerable community is affected by a natural or human-made hazard, resulting in significant damage and impact.

Wildfire: A wildfire is an unplanned and uncontrolled fire in an area with combustible vegetation. It can be called forest fire, bushfire, or wildland fire depending on the type of vegetation. Some ecosystems actually depend on wildfires. However, there is a distinction between wildfires and controlled burns carried out by humans. Controlled burns can sometimes turn into wildfires. Prescribed burns are often used in modern forest management to reduce risks and support natural forest cycles.

Natural resource: Natural resources are resources taken from nature that are used with minimal alterations. They include sunlight, atmosphere, water, land, minerals, vegetation, and wildlife. These resources possess various valued qualities, including commercial and industrial use, aesthetic value, scientific interest, and cultural significance.

Desert: A desert is a barren area with minimal precipitation, making it difficult for plant and animal life to survive. This exposes the ground to erosion. Approximately one-third of Earth's land surface is arid or semi-arid, including polar regions. Deserts can be classified based on precipitation, temperature, causes of desertification, or geographical location.

Oasis: An oasis is a fertile area in a desert or semi-desert where plants thrive and animals find shelter. It can have surface water or rely on wells and underground channels created by humans. Oases may also serve as historical rest stops on routes, offering access to underground water through wells.

Drought: A drought is an extended period of drier-than-normal conditions that can last for days, months, or years. It has significant impacts on ecosystems, agriculture, and the local economy. Dry seasons in the tropics increase the chances of drought and wildfires. Heat worsens drought conditions by accelerating evaporation, drying out vegetation, and increasing the risk of wildfires.

Shrubland: Shrubland is a plant community dominated by shrubs, often with grasses, herbs, and geophytes. It can occur naturally or result from human activity. It can be a mature vegetation type or a transitional community after a disturbance like fire. The stability may be maintained by regular disturbances or browsing. Shrubland can be unsuitable for human habitation due to the risk of fire. Coined in 1903.

Tundra: Tundra refers to a treeless biome hindered by frigid temperatures and short growing seasons. It derives from the Finnish word meaning "treeless plain". Three distinct regions exist: Arctic, alpine, and Antarctic tundra.

Forest: A forest is a land area with dominant tree presence. Definitions of forests vary worldwide, considering factors such as tree density, height, land use, legal standing, and ecological function. According to the Food and Agriculture Organization (FAO), a forest is land covering over 0.5 hectares, with trees higher than 5 meters and canopy cover exceeding 10 percent, excluding agricultural or urban land. In 2020, forests covered approximately 31 percent of the world's land, covering 4.06 billion hectares.

Rainforest: Rainforests are lush forests with unbroken tree canopies, consisting of moisture-reliant plants, epiphytes, and lianas, while lacking wildfires. They are categorized as tropical or temperate, with various other classifications existing.

Tropical rainforest: A tropical rainforest is a type of forest found within a specific climate zone, characterized by constant rainfall and no dry season. It is located between 10 degrees north and south of the equator. These rainforests are part of the larger tropical forest biome and are known for their lush vegetation and diverse wildlife. They are classified as tropical moist broadleaf forests and can be found in areas where the average precipitation is at least 60 mm.

Taiga: The taiga, also known as the boreal forest or snow forest, is a biome dominated by coniferous trees like pines, spruces, and larches. It is primarily found in North America and is characterized by its cold climate and extensive evergreen forests.

Temperate broadleaf and mixed forests: Temperate broadleaf and mixed forests refer to a terrestrial habitat type characterized by the World Wide Fund for Nature. It encompasses regions with broadleaf trees, mixed with conifer and broadleaf trees in coniferous forests.

Jungle: A jungle is a dense forest with tangled vegetation found in tropical climates. Its definition has changed over the past century.

Grassland: Grasslands are areas dominated by grasses, but also include sedges, rushes, and other plants. They exist on all continents except Antarctica, covering a significant portion of the Earth's land. Grasslands come in various types, including natural, semi-natural, and agricultural. They are among the largest biomes globally, making up 31-69% of the world's land area.

Meadow: A meadow is an open field with grasses and non-woody plants. It can have a few trees or shrubs but must remain open. Meadows can occur naturally or be created by clearing shrubs or woodlands. They are often used for hay, fodder, or livestock. These habitats are semi-natural grasslands with native species and limited human intervention.

Prairie: Prairies are temperate grassland ecosystems with moderate rainfall and dominated by grasses, herbs, and shrubs rather than trees. They are found mainly in North America, including the Great Plains, as well as in regions like Argentina, Brazil, Uruguay, Ukraine, Russia, and Kazakhstan. This biome encompasses the Interior Lowlands of Canada, the United States, and Mexico.

Savanna: A savanna is a type of ecosystem with a mix of woodlands and grasslands, characterized by widely spaced trees that do not form a closed canopy. The open canopy allows enough light to reach the ground, supporting a layer of grasses. There are four forms of savanna: savanna woodland with a light canopy of trees and shrubs, tree savanna with scattered trees and shrubs, shrub savanna with distributed shrubs, and grass savanna with minimal trees and shrubs.

Steppe: A steppe is a grassland region without dense forests, typically found near rivers and lakes. It includes montane, tropical/subtropical, and temperate grassland biomes.

Wetland: A wetland is a unique ecosystem that is flooded or saturated by water for extended periods. It is characterized by anoxic hydric soils and vegetation adapted to these conditions. Wetlands are highly biodiverse and support a wide range of plant and animal species. Assessment methods have been developed to conserve and protect wetlands worldwide. Constructed wetlands are purposefully designed to treat wastewater and manage stormwater in urban areas.

Bog: A bog is a type of wetland that collects peat from dead plants, usually mosses like sphagnum moss. It is one of the main types of wetlands and is also called mire, mosses, quagmire, or muskeg. Bogs can be found in forests and are often covered with heath or heather shrubs. They act as carbon sinks by gradually storing decayed plant material.

Mangrove: Mangroves are shrubs or trees that thrive in coastal saline or brackish water. They possess unique adaptations to absorb oxygen and eliminate salt, enabling them to handle harsh conditions. Mangroves are found in tropical regions worldwide, between 30° N and 30° S latitude, particularly near the equator. These taxonomically diverse plants emerged during the Late Cretaceous to Paleocene epochs and have spread due to tectonic plate movements. The earliest known mangrove palm fossils date back 75 million years.

Marsh: A marsh is a wetland dominated by herbaceous plants, not trees. It refers to low-lying and seasonally waterlogged areas and can include drained meadows and embanked polderlands.

Swamp: A swamp is a forested wetland that exists as a transition zone between land and water. They come in various sizes and are located worldwide, with fresh, brackish, or seawater. Freshwater swamps rely on rainwater and flooding to maintain water levels, while saltwater swamps are found in tropical and subtropical coasts. Swamps can have dry-land protrusions covered by water-tolerant vegetation. They can be classified as true forests or shrub swamps. In Canada, swamps are colloquially referred to as bogs, fens, or muskegs in boreal regions. Major rivers like the Amazon, Mississippi, and Congo have some of the world's largest swamps.

Climatology: Climatology, also known as climate science, studies Earth's climate by examining weather conditions averaged over 30 years or more. It explores climate variability, mechanisms of climate change, and modern climate change. Climatology is a subdivision of physical geography and is part of the atmospheric sciences. Additionally, it incorporates aspects of oceanography and biogeochemistry.

El Niño–Southern Oscillation: El Niño–Southern Oscillation (ENSO) is a weather phenomena that occurs irregularly, affecting winds and sea surface temperatures in the tropical eastern Pacific Ocean. It influences the climate in the tropics and subtropics. El Niño refers to a warming phase, while La Niña is the cooling phase. The Southern Oscillation is the atmospheric component linked to the sea temperature change. El Niño is associated with high air surface pressure in the tropical western Pacific, and La Niña with low air surface pressure there. These periods typically last several months and happen every few years with varying intensity. The mechanisms behind this oscillation are still being studied.

Köppen climate classification: The Köppen climate classification, developed by Wladimir Köppen, is a widely used system for classifying different climates. It has been modified over the years by Köppen and Rudolf Geiger, and is also known as the Köppen-Geiger climate classification.

Climate: Climate is the long-term weather pattern, encompassing mean and variability of meteorological factors such as temperature, humidity, pressure, wind, and precipitation. It is influenced by geographic factors like latitude, longitude, altitude, terrain, water bodies, and their currents. Climate integrates the components of the climate system, including the atmosphere, hydrosphere, cryosphere, lithosphere, and biosphere.

Temperate climate: Temperate climate refers to the weather conditions found in the middle latitudes, between the tropics and polar regions. It is characterized by wider temperature ranges and distinct seasonal changes compared to tropical climates, where variations are typically small and less noticeable.

Tropical climate: Tropical climate, classified as group A in the Köppen climate classification, is characterized by hot temperatures throughout the year and a minimum monthly average temperature of 18 °C (64.4 °F). It experiences abundant annual precipitation with a rhythmic seasonal pattern, although varying degrees of dryness may occur. This climate has two distinct seasons, a wet season and a dry season, and typically has a narrow annual temperature range. Additionally, tropical climates are known for their intense sunlight.

Climate change: Climate change refers to the ongoing global warming and its impact on Earth's climate system. This includes previous long-term changes to the climate. Human activities, such as burning fossil fuels and deforestation, contribute to the rapid increase in global temperatures. This is primarily due to the release of greenhouse gases like carbon dioxide and methane, which trap heat in Earth's atmosphere, resulting in global warming.

Climate variability and change: Climate variability encompasses all climate variations lasting longer than individual weather events, while climate change specifically encompasses persistent variations lasting decades or more. It refers to both historical and contemporary changes, with a notable focus on current global warming. Human activities, particularly since the Industrial Revolution, have significantly influenced the climate.

Greenhouse effect: The greenhouse effect is the trapping of heat by greenhouse gases in a planet's atmosphere, which raises its temperature. Stars emit shortwave radiation that passes through greenhouse gases, but planets emit longwave radiation that is partly absorbed by greenhouse gases. This reduces the rate at which a planet can cool off and adds to the planet's average surface temperature.

Season: A season is a period of the year characterized by changes in weather, daylight hours, and ecological patterns. These changes occur due to the Earth's tilted orbit around the Sun. The varying intensity of sunlight affects animal behavior and plant growth. Different cultures have different definitions of seasons, leading to variations in their number and nature.

Spring (season): Spring, or springtime, is one of the four temperate seasons. It follows winter and comes before summer. Its definition may vary depending on local climate and customs. In the Northern Hemisphere, spring coincides with autumn in the Southern Hemisphere. During spring equinox, day and night are roughly equal, but days lengthen and nights shorten as the season progresses until the Summer Solstice.

Summer: Summer is the hottest season, following spring and preceding autumn. It features the longest daylight hours and shortest darkness hours, with decreasing day length after the summer solstice. The earliest sunrises and latest sunsets happen during this time. The start of summer varies based on climate, tradition, and culture. Additionally, it is winter in the Southern Hemisphere while summer occurs in the Northern Hemisphere.

Autumn: Autumn, also known as fall, is one of the four temperate seasons and marks the transition from summer to winter. It is characterized by shorter daylight hours, cooler temperatures, and the iconic change in leaf color for deciduous trees.

Winter: Winter is the year's coldest and darkest season in polar and temperate climates. It follows autumn and precedes spring. Earth's axis tilt causes seasons, with winter happening when a hemisphere faces away from the Sun. The start of winter varies across cultures and can be based on weather.

Atmosphere of Earth: The Earth's atmosphere is a layer of gases surrounding the planet, kept in place by gravity. It creates pressure, shields us from meteoroids and harmful solar radiation, regulates temperature, and enables the existence of life and liquid water on the planet's surface.

Air mass: An air mass is a large volume of air defined by temperature and humidity, spanning hundreds or thousands of square miles. It adapts to the surface below it and is classified based on source region and latitude. Colder air masses are polar or arctic, while warmer ones are tropical. Continental and superior air masses are dry, while maritime and monsoon air masses are moist. Weather fronts separate air masses with different densities. As air masses move away from source regions, their characteristics can quickly change due to vegetation and water bodies. Classification schemes analyze an air mass's features and modifications.

Atmospheric pressure: Atmospheric pressure, also called air pressure or barometric pressure, refers to the pressure within the Earth's atmosphere. It can be measured using various units such as pascal (Pa), hectopascal (hPa), millibar (mbar), millimeter of mercury (mm Hg), or pounds per square inch (psi). The standard atmosphere is defined as 101,325 Pa, which roughly corresponds to the average sea-level atmospheric pressure on Earth.

Atmospheric science: Atmospheric science is the study of the Earth's atmosphere and its physical processes. It includes meteorology for weather forecasting, climatology for studying climate changes, and aeronomy for the upper layers of the atmosphere. It has also expanded to include planetary science and the study of atmospheres in the Solar System.

Ozone depletion: Ozone depletion refers to the decrease in Earth's atmospheric ozone, with two notable events observed since the late 1970s. The first is a gradual four percent decline in total ozone levels globally, while the second is a significant reduction in stratospheric ozone specifically around the polar regions during spring, known as the ozone hole. Additionally, springtime polar tropospheric ozone depletion events occur alongside these stratospheric events.

Aurora: Aurora, known as the northern or southern lights, is a captivating natural light phenomenon visible in high-latitude areas. These displays showcase stunning patterns of radiant curtains, rays, spirals, and flickering lights that illuminate the entire sky.

Mirage: A mirage is a natural optical phenomenon where light rays bend, creating a displaced image of faraway objects or the sky. It derives from the French word "mirer" and Latin word "mirari," meaning "to look at, to wonder at."

Rainbow: A rainbow, caused by light's refraction, reflection, and dispersion in water droplets, is a multicolored circular arc in the sky. It appears opposite the Sun and can be formed by rain, mist, spray, or dew.

Sky: The sky refers to the view upward from Earth's surface, encompassing the atmosphere, outer space, and serving as a distinct realm between the ground and outer space.

Meteorology: Meteorology is the study of weather forecasting and atmospheric sciences. Its origins date back to ancient times, but significant progress began in the 18th century. The 19th century saw advancements through weather observation networks. Historical data was previously relied upon for weather prediction. Breakthroughs in weather forecasting happened in the latter half of the 20th century with the development of computers and understanding of physics. Marine weather forecasting is an important branch, considering maritime and coastal safety, along with atmospheric interactions and large bodies of water.

Weather: Weather is the condition of the atmosphere, encompassing temperature, precipitation, and other elements. It determines whether it is hot or cold, wet or dry, calm or stormy, and clear or cloudy. Weather pertains to day-to-day atmospheric conditions, mainly occurring in the troposphere. Climate refers to longer-term averages of these conditions. "Weather" typically refers to Earth's atmospheric conditions.

Weather front: A weather front is a boundary that separates air masses with different characteristics like density, wind, temperature, and humidity. It often causes disturbed and unstable weather, leading to thunderstorms, precipitation, fog, and wind shifts. Dry lines in summer can trigger severe weather, while some fronts may not produce precipitation or cloudiness.

Weather forecasting: Weather forecasting is the scientific prediction of atmospheric conditions at a specific place and time. It has been practiced both informally for centuries and formally since the 19th century.

Humidity: Humidity refers to the amount of water vapor in the air. It is not visible to the naked eye, but plays a role in determining the possibility of precipitation, dew, or fog.

Low-pressure area: A low-pressure area is a region with lower atmospheric pressure than its surroundings. It typically brings bad weather, while high-pressure areas have calmer conditions. In the northern hemisphere, winds circle counterclockwise around lows due to Coriolis forces. Low-pressure systems form when wind divergence occurs in the upper atmosphere, known as cyclogenesis. Divergence can occur on the east side of upper troughs or ahead of smaller wavelength embedded shortwave troughs.

Cloud: A cloud is a visible mass of tiny liquid droplets, frozen crystals, or particles suspended in the atmosphere. It can form when air is cooled to its dew point or gains enough moisture to reach the ambient temperature. Water and other chemicals can make up the droplets and crystals.

Cirrus cloud: Cirrus clouds are high, ice crystal clouds that appear delicate and wispy with white strands. They form when warm, dry air rises and water vapor deposits onto dust particles at high altitudes. Found globally between 4,000 and 20,000 meters above sea level, with higher elevations in the tropics and lower elevations in polar regions.

Cumulonimbus cloud: A cumulonimbus cloud is a dense, towering vertical cloud that forms from condensation of water vapor in the lower troposphere. It carries upward by powerful air currents and turns into ice crystals, causing hail and lightning. These thunderheads can form alone, in clusters, or along squall lines, and they can produce severe weather like tornadoes, strong winds, and large hailstones. They develop from overdeveloped cumulus congestus clouds and are abbreviated as Cb.

Cumulus cloud: Cumulus clouds are low-level clouds with a flat base and a puffy, cotton-like appearance. The name "cumulus" comes from Latin, meaning "heap" or "pile". They typically form below 2,000 m in altitude and can appear individually, in groups, or in lines.

Fog: Fog is a misty cloud formed by small water droplets or ice crystals that hovers close to the Earth's surface. It often resembles stratus clouds and is influenced by water bodies, terrain, and wind. Fog significantly impacts activities like shipping, travel, and warfare.

Precipitation: Precipitation, in meteorology, refers to the condensation of atmospheric water vapor that falls from clouds due to gravity. It takes various forms such as drizzle, rain, sleet, snow, ice pellets, graupel, and hail. Precipitation occurs when the atmosphere becomes saturated with water vapor, leading to its condensation and subsequent falling. This process is distinct from fog and mist, as they do not condense enough to precipitate. Air can become saturated through cooling or adding water vapor. Precipitation forms when smaller droplets merge with other raindrops or ice crystals in a cloud. Showers are brief intense periods of rain occurring in scattered locations.

Rain: Rain is condensed water vapor that falls to the ground due to gravity. It plays a vital role in the water cycle by providing most of Earth's fresh water and supporting hydroelectric power, crop irrigation, and various ecosystems.

Acid rain: Acid rain is unusually acidic precipitation with elevated levels of hydrogen ions. It has a lower pH (4-5 avg.) compared to the neutral pH of most water (6.5-8.5). Acid rain harms plants, aquatic animals, and infrastructure. It occurs due to emissions of sulfur dioxide and nitrogen oxide reacting with atmospheric water molecules, forming acids.

Snow: Snow is formed by ice crystals suspended in the atmosphere that grow and fall to the ground, undergoing changes. It is composed of frozen crystalline water throughout its life cycle, starting as ice crystals in the atmosphere, falling and accumulating on surfaces, transforming in place, and eventually melting or disappearing.

Hail: Hail is a type of solid precipitation, made up of ice balls or irregular lumps called hailstones. It is different from ice pellets, though they are often mistaken for each other. Hailstones form and fall in warmer temperatures compared to ice pellets, as low surface temperatures hinder their growth.

Dew: Dew is moisture that forms as droplets on objects, typically in the morning or evening, when the surface temperature cools and condensation occurs.

Frost: Frost is a thin layer of ice that forms when water vapor deposits onto a freezing surface. It occurs when the air has excess water vapor for a specific temperature, similar to dew formation but below the freezing point of water.

Atmospheric circulation: Atmospheric circulation refers to the movement of air on a large scale, playing a crucial role in redistributing thermal energy across the Earth's surface. While it varies annually, the fundamental structure remains consistent. While smaller weather systems are unpredictable, making long-range predictions beyond ten days in practice or a month in theory is challenging.

Anticyclone: An anticyclone is a weather pattern characterized by wind circulation around a central area of high atmospheric pressure. In the Northern Hemisphere, winds rotate clockwise, while in the Southern Hemisphere, they rotate counterclockwise. Anticyclones bring clear skies, cooler temperatures, and drier air. Additionally, they can cause fog to form overnight.

Cyclone: A cyclone is a large air mass with low atmospheric pressure that rotates around a strong center. It has inward-spiraling winds and rotates counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. Cyclones can be polar vortices, extratropical cyclones, tropical cyclones, subtropical cyclones, mesocyclones, tornadoes, or dust devils.

Extratropical cyclone: Extratropical cyclones, also known as mid-latitude cyclones or wave cyclones, are low-pressure systems that play a significant role in shaping the Earth's weather. They can range from producing light showers to extreme weather events like gales, thunderstorms, blizzards, and tornadoes. These cyclones occur in the middle latitudes and cause rapid temperature and dew point changes along weather fronts.

Hadley cell: The Hadley cell is a tropical atmospheric circulation that involves air rising near the equator, flowing poleward near the tropopause, and descending in the subtropics. It is driven by differences in heating between the tropics and subtropics. The circulation is characterized by cells on both sides of the equator, with the Southern Hemisphere cell slightly stronger. During summer and winter, a single, cross-equatorial cell dominates. Similar circulations may exist on Venus and Mars.

Polar vortex: A polar vortex is a large, cold, rotating region of air that encircles Earth's polar regions. It exists in both the stratosphere and troposphere, rotating in the same direction as Earth but with different sizes, structures, seasonal cycles, and impacts on weather. It is also found on other low-obliquity rotating planets.

Walker circulation: The Walker circulation, also known as the Walker cell, is a model of air flow in the tropics. It describes a closed circulation of air in the zonal and vertical directions caused by heat differences between the ocean and land. This circulation is consistent with observations and is part of the larger Hadley Circulation, which also includes motion in the meridional direction.

Storm: A storm is a disturbed state of the environment or atmosphere that causes significant disruptions, such as strong winds, tornadoes, hail, thunder, lightning, and heavy precipitation. It can also involve freezing rain, dust storms, and other severe weather conditions.

Blizzard: A blizzard is a severe snowstorm with strong winds and poor visibility that lasts for several hours. Ground blizzards occur when loose ground snow is lifted and blown by strong winds. These storms can be massive, stretching over hundreds or thousands of kilometers.

Tornado: A tornado is a violently rotating column of air that connects the Earth's surface with a cumulonimbus cloud. It is commonly called a twister or whirlwind. Tornadoes vary in size and shape, often appearing as a funnel-shaped cloud with debris below. Most tornadoes have wind speeds under 180 km/h, but the most extreme ones can exceed 480 km/h. They typically travel several kilometers before dissipating but can persist over 100 km.

Tropical cyclone: A tropical cyclone is a rapidly rotating storm system characterized by a low-pressure center, strong winds, and heavy rain. It has a spiral arrangement of thunderstorms and can be called different names depending on its location and strength, such as hurricane, typhoon, or tropical storm. It is referred to as a hurricane in the Atlantic and northeastern Pacific Ocean, and a typhoon in the northwestern Pacific Ocean. In the Indian Ocean and South Pacific, it is called a tropical cyclone or severe cyclonic storm.

Dust storm: A dust storm, also known as a sandstorm, occurs in arid and semi-arid areas when strong winds sweep loose sand and dirt from dry surfaces. This process, called saltation and suspension, transports fine particles by depositing them in different locations.

Lightning: Lightning is a natural phenomenon of electrostatic discharges that occur between charged regions in the atmosphere or between the atmosphere and the ground. These discharges release a high amount of energy, ranging from 200 megajoules to 7 gigajoules, and generate various forms of electromagnetic radiation, including visible light. Thunder, caused by the shock wave created during lightning, is a common outcome. Lightning is predominantly associated with thunderstorms but can also occur during volcanic eruptions, and it plays a role in the global atmospheric electrical circuit.

Thunderstorm: A thunderstorm or electrical storm is a weather phenomenon characterized by lightning and thunder. It occurs in cumulonimbus clouds and can be accompanied by strong winds and heavy precipitation, including snow, sleet, or hail. Some thunderstorms form a line or squall line, while others rotate like cyclones, known as supercells. Severe thunderstorms can produce dangerous weather conditions like large hail, strong winds, and even tornadoes. Vertical wind shear can cause deviations in their course.

Wind: Wind is the movement of air or gases on a planet's surface. It occurs on various scales, from short-term thunderstorm flows to longer-lasting local and global winds. These winds are primarily caused by differential heating and the planet's rotation. Factors such as terrain, plateaus, and coastlines can also influence wind patterns.

Jet stream: The jet stream is a fast-flowing, narrow air current found in the atmospheres of several planets. On Earth, it is located near the tropopause and moves from west to east. It can change direction, split into multiple streams, or combine into a single current.

Monsoon: A monsoon is a seasonal wind pattern that brings changes in rainfall. It is associated with the movement of the Intertropical Convergence Zone between the northern and southern limits of the equator. The term is mainly used to refer to the rainy phase of the seasonal pattern, but there is also a dry phase. Occasionally, it can describe intense but brief local rainfall events.

Trade winds: The trade winds are permanent east-to-west prevailing winds in the equatorial region of Earth. They blow from the northeast in the Northern Hemisphere and from the southeast in the Southern Hemisphere. Trade winds have been utilized by sailors for centuries to navigate and explore the world's oceans, facilitating European colonization of the Americas and the establishment of trade routes across the Atlantic and Pacific Oceans.

Geology: Geology is the scientific study of the Earth, rocks, and the processes that shape them over time. It is closely related to other Earth sciences and is connected to Earth system science and planetary science.

Internal structure of Earth: The Earth's internal structure is composed of layers: a solid crust, viscous asthenosphere, solid mantle, liquid outer core, and solid inner core. The flow of the outer core generates the Earth's magnetic field. It excludes the atmosphere and hydrosphere.

Earth's inner core: The Earth's inner core is the innermost layer of our planet, which is a solid sphere measuring about 1,220 km (760 mi) in radius. It constitutes around 20% of Earth's total radius or about 70% of the Moon's radius.

Earth's outer core: The Earth's outer core is a fluid layer made up mostly of iron and nickel. It is about 2,260 km thick and located between the solid inner core and the mantle. Starting at the core-mantle boundary, it extends 2,889 km below the Earth's surface and ends 5,150 km deep at the inner core boundary.

Mantle (geology): The mantle in geology refers to a layer between a planet's core and crust. It consists of rock or ices and is the largest and most massive layer in the planet. Mantles are found in planetary bodies that have undergone density differentiation. They are present in terrestrial planets, certain asteroids, and some planetary moons.

Geophysics: Geophysics is a scientific field that explores the physical processes and properties of the Earth and its surroundings. Geophysicists use quantitative methods to analyze various aspects of the planet's structure, composition, dynamics, and surface expressions. It encompasses solid earth applications, such as studying Earth's shape, gravitational and magnetic fields, as well as its internal structure. However, the definition has expanded to include a broader range, spanning the water cycle, fluid dynamics of oceans and atmosphere, electricity and magnetism in different atmospheric layers, solar-terrestrial physics, and even celestial bodies like the Moon and other planets.

Earth's magnetic field: Earth's magnetic field, or geomagnetic field, is a magnetic force field extending from Earth's interior into space. It interacts with the solar wind and is generated by electric currents caused by the motion of molten iron and nickel in Earth's outer core. This occurs as heat escapes from the core, creating a geodynamo.

Geodesy: Geodesy is the study of measuring and representing Earth's changing 3D geometry, gravity, and spatial orientation. It also encompasses planetary geodesy, which examines similar aspects of other astronomical bodies.

Seismology: Seismology is the study of earthquakes and how elastic waves travel through the Earth. It examines various seismic sources like volcanoes, tectonic activities, and human-induced processes. Seismologists also study environmental effects such as tsunamis. Paleoseismology uses geology to understand past earthquakes. Seismograms, created by seismographs, record Earth's motion over time. Seismologists are scientists specializing in this field.

Earthquake: Summary: An earthquake, also known as a quake or tremor, occurs when energy is suddenly released in the Earth's lithosphere, causing seismic waves. They vary in intensity, from imperceptible to destructive, damaging infrastructure and propelling objects and people. Seismic activity refers to the frequency, type, and size of earthquakes in a specific area over time. Seismicity is the average rate of energy release in a particular location. The term tremor can also describe non-earthquake seismic rumbling.

Geological history of Earth: The geological history of Earth is the study of major events that have occurred over the planet's 4.54 billion year existence. It is based on the study of rock layers and follows a chronological measurement system called stratigraphy. Earth formed through the accumulation of dust and gas from the solar nebula, which also gave rise to the rest of the Solar System.

Geochronology: Geochronology is the science of determining the age of rocks and fossils using their inherent signatures. Radioactive isotopes enable absolute geochronology, while relative geochronology relies on tools like paleomagnetism and stable isotope ratios. Combining multiple indicators improves the precision of age determination.

Great Oxidation Event: The Great Oxidation Event (GOE) was a period in Earth's history around 2.460-2.426 billion years ago, when the atmosphere and shallow ocean experienced a significant increase in oxygen concentration. This event, also known as the Oxygen Catastrophe or Oxygen Revolution, transformed the atmosphere from a weakly reducing state to an oxidizing one. This change occurred due to the accumulation of biologically produced molecular oxygen, gradually raising oxygen levels to about 10% of today's atmosphere by the end of the GOE.

Cambrian explosion: The Cambrian explosion was a sudden and rapid increase in the number of complex life forms, occurring approximately 538.8 million years ago. This 13-25 million-year period marked the appearance of almost all major animal phyla in the fossil record, leading to the diversification of modern metazoan phyla. It also triggered significant diversification in other groups of organisms.

Late Ordovician mass extinction: The Late Ordovician mass extinction (LOME) occurred 445 million years ago and is considered the second-largest extinction event in terms of the percentage of genera lost. It affected all major taxonomic groups, causing the extinction of 49-60% of marine genera and nearly 85% of marine species. Numerous marine organisms, such as brachiopods, bryozoans, conodonts, trilobites, echinoderms, corals, bivalves, and graptolites, disappeared. Despite its severity, it didn't significantly alter ecosystem structures or lead to major morphological innovations. Biodiversity gradually recovered over the first 5 million years of the subsequent Silurian period.

Late Devonian extinction: The Late Devonian extinction, one of the five largest mass extinctions in Earth's history, refers to two major extinction events. The first event, known as the Kellwasser event or Frasnian-Famennian extinction, happened around 372 million years ago, causing the extinction of 19% of all families and 50% of all genera. The second event, called the Hangenberg event or end-Devonian extinction, occurred 359 million years ago, marking the end of the Famennian and Devonian periods and the transition into the Carboniferous Period.

Permian–Triassic extinction event: The Permian-Triassic extinction event occurred about 251.9 million years ago, marking the boundary between the Permian and Triassic periods. It is the most severe known extinction event on Earth, resulting in the extinction of 57% of biological families, 83% of genera, 81% of marine species, and 70% of terrestrial vertebrate species. This event also led to the largest mass extinction of insects. It is considered the largest of the "Big Five" mass extinctions, and there is evidence for multiple distinct pulses or phases of extinction.

Triassic–Jurassic extinction event: The Triassic-Jurassic Extinction Event (TJME) occurred 201.4 million years ago, marking the shift between the Triassic and Jurassic periods. It is ranked among the top five major extinction events in history, significantly impacting life both on land and in the oceans. Many marine species, including the entire class of conodonts and 23-34% of marine genera, vanished. On land, all archosauromorphs (excluding crocodylomorphs, pterosaurs, and dinosaurs) became extinct, along with previously abundant groups like aetosaurs, phytosaurs, and rauisuchids. Some terrestrial animals had already disappeared before the event, but there is uncertainty due to limited fossils from the Late Triassic. Notably, plants, crocodylomorphs, dinosaurs, pterosaurs, and mammals were largely unaffected. This allowed dinosaurs, pterosaurs, and crocodylomorphs to dominate the land for the next 135 million years.

Cretaceous–Paleogene extinction event: The Cretaceous-Paleogene extinction event, also known as the K-T extinction, occurred approximately 66 million years ago and caused the extinction of 75% of plant and animal species on Earth. This event led to the complete extinction of non-avian dinosaurs and most large tetrapods, except for some ectothermic species. It marked the end of the Cretaceous period and the Mesozoic era, while ushering in the Cenozoic era, which continues today.

Pangaea: Pangaea was a supercontinent that formed 335 million years ago from Gondwana, Euramerica, and Siberia. It began to break apart 200 million years ago, with its landmass centered on the equator. Pangaea was surrounded by the superocean Panthalassa and the Paleo-Tethys and subsequent Tethys Oceans. It is the most recent supercontinent and the first to be reconstructed by geologists.

Geologic time scale: The geologic time scale is a chronological dating system used by Earth scientists to describe the timing and relationships of events in Earth's history based on the rock record. It is developed through the study of rock layers and their identifying features like fossils and paleomagnetic properties. The International Commission on Stratigraphy is responsible for defining standardized international units of geologic time, which are used to define divisions of geologic time.

Precambrian: The Precambrian is the earliest part of Earth's history, accounting for 88% of its geologic time. It preceded the Phanerozoic Eon and is named after its successor, the Cambrian period. Rocks from this age were first studied in Wales.

Phanerozoic: The Phanerozoic is the most recent eon in Earth's geologic time scale, spanning from 538.8 million years ago to present. It is known for the proliferation and diversification of animal and plant life, which colonized different habitats on Earth's surface. It started with the Cambrian period, when animals developed hard shells that left clear fossil records. The time preceding the Phanerozoic is referred to as the Precambrian and is divided into the Hadean, Archaean, and Proterozoic eons.

Paleozoic: The Paleozoic Era is the first of three geological eras in the Phanerozoic Eon, lasting from 538.8 to 251.9 million years ago. It follows the Neoproterozoic era and precedes the Mesozoic Era. The Paleozoic is divided into six geologic periods.

Cambrian: The Cambrian Period, part of the Paleozoic Era, marks the beginning of the Phanerozoic Eon. Lasting about 53.4 million years, it started after the Ediacaran Period around 538.8 million years ago and ended at the start of the Ordovician Period 485.4 million years ago. Important details regarding its subdivisions and base remain somewhat uncertain.

Permian: The Permian is a geologic period that lasted 47 million years, marking the end of the Paleozoic Era and the beginning of the Mesozoic Era. It occurred between 298.9 and 251.902 million years ago. The term was coined by Sir Roderick Murchison in 1841, naming it after the Perm region in Russia.

Mesozoic: The Mesozoic Era, lasting from 252 to 66 million years ago, was a period characterized by the dominance of gymnosperms and dinosaurs. It had a hot greenhouse climate and marked the tectonic break-up of Pangaea. The Mesozoic Era was the middle era since complex life evolved, sandwiched between the Paleozoic and the Cenozoic.

Triassic: The Triassic is a geologic period lasting 50.5 million years, bridging the gap between the Permian and Jurassic periods. It marks the start of the Mesozoic Era and is characterized by major extinction events. Divided into Early, Middle, and Late epochs, it is the first and shortest period of the Mesozoic Era.

Jurassic: The Jurassic, from 201.4 to 145 million years ago, is a significant geologic period within the Mesozoic Era. It is associated with the Jura Mountains, where limestone strata from this period were first recognized.

Cretaceous: The Cretaceous, the longest period in the Mesozoic Era, lasted from about 145 to 66 million years ago. It is characterized by its abundance of chalk and is abbreviated as K.

Cenozoic: The Cenozoic is the current geological era, covering the past 66 million years. It is characterized by the dominance of mammals, birds, and angiosperms. It follows the Mesozoic and Paleozoic eras and began with the extinction event that wiped out non-avian dinosaurs, primarily caused by the impact of a large asteroid called the Chicxulub impactor.

Paleogene: The Paleogene is a geologic period lasting 43 million years, starting from the end of the Cretaceous Period 66 million years ago to the beginning of the Neogene Period 23.03 million years ago. It marks the start of the Cenozoic Era in the current Phanerozoic Eon. The Paleogene was previously referred to as the Tertiary Period and is sometimes still informally called that. It is commonly abbreviated as "Pg".

Neogene: The Neogene is a geological period that lasted for about 20 million years, starting from 23 million years ago and ending about 2.5 million years ago. It is divided into two epochs: the Miocene and the Pliocene. Some geologists argue that it is challenging to distinguish the Neogene from the Quaternary period. The term "Neogene" was coined in 1853 by Moritz Hörnes, an Austrian paleontologist. Previously, the Tertiary Period was used to describe the time period that now includes both the Paleogene and Neogene, although it is no longer an officially recognized term.

Quaternary: The Quaternary is the most recent period of the Cenozoic Era, beginning 2.58 million years ago. It consists of two epochs, the Pleistocene and the Holocene, but the proposed Anthropocene epoch is not yet officially recognized.

Holocene: The Holocene is the present geological epoch that started about 9,700 years ago. It follows the Last Glacial Period and marks the retreat of glaciers. It is part of the Quaternary period and corresponds to the current warm period known as MIS 1. Some consider it an interglacial period within the Pleistocene Epoch, referred to as the Flandrian interglacial.

Mineral: A mineral is a naturally occurring solid substance with a distinct chemical composition and crystal structure. It is found in its pure form and is studied in geology and mineralogy.

Mineralogy: Mineralogy is a branch of geology that focuses on the scientific study of minerals and mineralized artifacts. It includes analyzing their chemical composition, crystal structure, and physical properties. Specific areas of study in mineralogy include understanding how minerals form, classifying them, determining their distribution across various locations, and exploring their practical uses.

Gemstone: Gemstones are mineral crystals used for making jewelry. Some non-mineral materials are also considered gemstones. They can be hard or soft, chosen for their aesthetics. Rarity and fame add value to gemstones.

Sapphire: Sapphire is a precious gem derived from corundum, usually blue but available in other colors like yellow, purple, orange, and green. It is also used in non-ornamental applications due to its hardness, including infrared optical components and electronic wafers. Sapphires are often cut into gemstones for jewelry and can be artificially created. They are the birthstone for September and commemorate the 45th anniversary. A sapphire jubilee takes place after 65 years.

Ruby: Ruby is a durable, pinkish red gemstone made of corundum. It is a popular choice for traditional jewelry and is part of the cardinal gems. The color is attributed to chromium, and the word 'ruby' is derived from the Latin term for red.

Asbestos: Asbestos is a natural mineral made up of thin fibrous crystals. When disturbed, these fibers can be released into the air and inhaled, causing serious lung conditions like mesothelioma, asbestosis, and lung cancer. Because of these health risks, asbestos is considered a significant hazard to health and safety.

Emerald: Emerald is a green gemstone formed from beryl, containing trace amounts of chromium or vanadium. It has a hardness of 7.5-8 on the Mohs scale and is known for its poor toughness due to trapped materials during formation. Emerald is classified as a cyclosilicate.

Pyroxene: Pyroxene minerals are essential components of many types of rocks. They have a chemical formula XY(Si,Al)2O6, with X representing Ca, Na, Fe, Mg, and sometimes other elements like Zn, Mn, or Li, and Y representing smaller ions like Cr, Al, Mg, Co, Mn, Sc, Ti, V, or Fe. Unlike other silicate minerals, pyroxenes have limited substitution of aluminum for silicon. They possess a common structure of single chains of silica tetrahedra. Clinopyroxenes crystallize in the monoclinic system, while orthopyroxenes crystallize in the orthorhombic system.

Jade: Jade is a type of ornamental rock used for jewelry and ornaments. It is categorized into two types: nephrite and jadeite. Nephrite is typically green but can also be yellow, white, or black. Jadeite comes in various colors, including green, lavender, yellow, orange, brown, and rarely blue. However, the names "nephrite" and "jadeite" are technically incorrect as they refer to rock aggregates rather than specific minerals. Nephrite was officially replaced as a mineral species name in 1978, while jadeite is a legitimate mineral species. In China, the traditional name for jadeite is "fei cui," which predates its official name.

Mica: Mica is a group of silicate minerals known for their ability to split into thin, elastic plates. It is commonly found in igneous and metamorphic rocks, with occasional flakes in sedimentary rock. Mica is prominent in granites, pegmatites, and schists, and massive mica formations have been discovered in certain pegmatites.

Kaolinite: Kaolinite, also known as kaolin, is a clay mineral composed of Al2Si2O5(OH)4. It is a layered silicate mineral with one sheet of silica connected to one sheet of alumina through oxygen atoms.

Talc: Talc is a clay mineral containing hydrated magnesium silicate (Mg3Si4O10(OH)2). It is commonly used in powdered form, often mixed with corn starch, as baby powder. Talc is also used as a thickening agent, lubricant, and important ingredient in ceramics, paints, and roofing materials. It is widely used in cosmetics and can be found as foliated to fibrous masses or in a rare crystal form. Talc has a perfect basal cleavage, an uneven flat fracture, and a two-dimensional platy structure.

Quartz: Quartz is a hard mineral made of silicon dioxide. Its atoms form a continuous framework of silicon-oxygen tetrahedra, resulting in its chemical formula, SiO2. Quartz is Earth's second most abundant mineral and is found in the continental crust, second only to feldspar.

Feldspar: Feldspar is a group of minerals that form rocks and contain aluminium tectosilicate. They commonly have other cations like sodium, calcium, or potassium. The group includes plagioclase and alkali feldspars. Feldspars make up around 60% of the Earth's crust and 41% of the continental crust by weight.

Gypsum: Gypsum is a soft mineral containing calcium sulfate dihydrate (CaSO4·2H2O). It is extensively mined and commonly used in fertilizers, plaster, drywall, and chalk. Gypsum can form translucent selenite crystals and is created through the evaporation of minerals or as a hydrated product of anhydrite. Its hardness is rated 2 on the Mohs scale.

Petrology: Petrology is a branch of geology that focuses on the study of rocks and the conditions of their formation. It comprises three subdivisions: igneous, metamorphic, and sedimentary petrology. The former two are often taught together due to their reliance on chemistry and phase diagrams. Conversely, sedimentary petrology is taught alongside stratigraphy as it explores the processes involved in forming sedimentary rocks. Modern sedimentary petrology is increasingly incorporating chemistry into its methodologies.

Rock (geology): Rocks are naturally occurring solid masses or aggregates made up of minerals or mineraloid matter. They vary in composition, formation, and mineral content. Rocks make up the Earth's crust and most of its interior, excluding the liquid outer core and pockets of magma. Studying rocks involves various aspects of geology, such as petrology and mineralogy. This field can encompass rocks found on Earth or include planetary geology, examining rocks from other celestial objects.

Ore: Ore is rock or sediment with valuable minerals above normal levels, usually metals, that can be profitably mined and sold. The concentration of desired materials in ore determines its grade. The value of minerals must surpass extraction costs to be considered ore. Complex ore contains multiple valuable minerals.

Igneous rock: Igneous rock is a primary rock type, formed by magma or lava cooling and solidifying. It is one of the three main rock types, alongside sedimentary and metamorphic rocks.

Basalt: Basalt is a common extrusive igneous rock found on Earth, as well as on other planets and moons in the Solar System. It forms from rapidly cooled lava with high magnesium and iron content. About 90% of volcanic rock on Earth is basalt. It is chemically similar to gabbro, a coarse-grained rock. Geologists observe basalt lava eruptions at around 20 volcanoes per year. Basalt covers a significant portion of Venus and the lunar maria, while also being present on the surface of Mars.

Granite: Granite is a common type of igneous rock found in the Earth's crust. It is composed mainly of quartz, alkali feldspar, and plagioclase. Granite forms underground from slowly cooling magma with high silica and alkali metal content. It can be found in various sizes, ranging from small dikes to large batholiths spanning hundreds of square kilometers.

Tuff: Tuff is a solid rock formed from volcanic ash expelled during eruptions. It is classified as tuff if it contains over 75% ash, and tuffaceous if it has 25% to 75% ash. Tuff composed of sandy volcanic material is known as volcanic sandstone.

Magma: Magma is molten material beneath Earth's surface, forming igneous rocks. It exists on other planets and satellites too. Apart from molten rock, magma can contain crystals and gas bubbles.

Metamorphic rock: Metamorphic rocks form when existing rocks undergo metamorphism, a process that involves high temperatures and pressures. This results in significant physical and chemical changes, leading to the formation of a new rock with different minerals and texture. The original rock, known as a protolith, can be igneous, sedimentary, or another metamorphic rock.

Schist: Schist is a metamorphic rock with a medium grain size and distinct schistosity. It splits easily into thin flakes due to its high content of platy minerals like mica, talc, chlorite, or graphite. These minerals are alternated with granular minerals like feldspar or quartz.

Marble: Marble is a metamorphic rock formed from carbonate minerals, such as calcite or dolomite, due to heat and pressure. It has a crystalline texture and is usually not layered, except for rare cases.

Gneiss: Gneiss is a prevalent metamorphic rock resulting from intense heat and pressure acting on igneous or sedimentary rocks. It develops at higher temperatures and pressures compared to schist. Notably, gneiss exhibits a distinct banded texture with alternating darker and lighter colored bands, lacking cleavage.

Slate: Slate is a metamorphic rock formed from shale or volcanic ash through low-grade metamorphism. It has a fine-grained, homogeneous structure and is the finest-grained foliated metamorphic rock. Its foliation occurs perpendicular to the direction of compression, which may not align with the original sedimentary layering.

Sedimentary rock: Sedimentary rocks are formed by the accumulation of mineral or organic particles on Earth's surface, followed by cementation. These particles, called sediment, come from weathering, erosion, volcanic activity, or dead organisms. Agents like water, wind, ice, or mass movement transport the sediment to its deposition location. Sedimentation can also occur when minerals precipitate from water solution.

Limestone: Limestone is a carbonate sedimentary rock and the main source of lime. It is composed of calcite and aragonite, crystal forms of CaCO3. Limestone forms when these minerals precipitate from calcium-rich water. This can occur through biological and nonbiological processes, with biological accumulation being significant for the past 540 million years. Limestone often contains fossils, providing insights into ancient environments and the evolution of life.

Oil shale: Oil shale is a sedimentary rock rich in kerogen, from which liquid hydrocarbons can be extracted. It also contains inorganic substances and bitumens. Oil shales are categorized as marine, lacustrine, or terrestrial based on their deposition environment. They differ from oil-bearing shales, which hold petroleum and are drilled for extraction. Oil shale should not be confused with tight oil, often referred to as shale oil.

Sandstone: Sandstone is a sedimentary rock consisting of sand-sized silicate grains, making up around 20–25% of all sedimentary rocks.

Conglomerate (geology): Conglomerate is a type of sedimentary rock made up of round to subangular gravel-sized fragments called clasts. It usually contains finer-grained sediments like sand, silt, or clay that fill the spaces between the clasts. The clasts and matrix are held together by substances like calcium carbonate, iron oxide, silica, or hardened clay.

Breccia: Breccia is a rock formed from large angular pieces of minerals or rocks, held together by a fine-grained matrix.

Bauxite: Bauxite is a sedimentary rock with a high aluminum content, serving as the primary global source for aluminum and gallium. It comprises aluminum minerals such as gibbsite, boehmite, and diaspore, along with iron oxides, kaolinite clay, and traces of anatase and ilmenite. Bauxite is characterized by its dull luster and reddish-brown, white, or tan color.

Meteorite: A meteorite is a piece of debris from outer space that survives its journey through the atmosphere to land on a planet or moon's surface. When it enters the atmosphere, it heats up, becoming a meteor and creating a fireball known as a shooting star. The brightest ones are called bolides by astronomers. Once it lands, it becomes a meteorite, which can vary in size. Geologists define a bolide as a meteorite that creates an impact crater.

Sediment: Sediment is natural material that undergoes weathering and erosion, and is then moved by wind, water, or ice, or by gravity. It can be carried in rivers and deposited as sediment on the sea bed. Over time, buried sediment can become rocks like sandstone and siltstone through a process called lithification.

Clay: Clay is a fine-grained soil material containing clay minerals that develop plasticity when wet and harden when fired. While most clay minerals are white or light-colored, natural clays can exhibit various colors due to impurities like iron oxide.

Gravel: Gravel is a natural loose collection of rock fragments created by erosion and sedimentary processes. It is commonly found on Earth and also commercially produced as crushed stone.

Sand: Sand is a granular material made up of small mineral particles. It comes in different compositions but is characterized by its size, larger than silt but smaller than gravel. Sand can also refer to a type of soil consisting of over 85% sand-sized particles.

Sedimentary basin: A sedimentary basin is a large depression in the Earth's crust where sediments have accumulated, forming a three-dimensional body of sedimentary rock. These basins are created by subsidence over long periods of time, providing space for the accumulation of sediments. The sediments undergo compaction and lithification during burial, transforming into sedimentary rock.

Soil science: Soil science is the study of the Earth's surface soil as a natural resource, which includes examining the formation, classification, and mapping of soil. It encompasses the study of the physical, chemical, biological, and fertility properties of soils and their relation to soil use and management.

Soil: Soil, often called earth or dirt, is a complex mixture of organic matter, minerals, gases, liquids, and organisms. It provides essential support for plants and soil organisms, distinguishing it from displaced soil, known as dirt.

Peat: Peat is organic matter formed in wetland areas, like bogs or mires. It is made up of partially decayed vegetation, with sphagnum moss being a common component. The mosses' biological features aid in peat formation by creating a suitable habitat. Soils consisting mostly of peat are called histosols. Peat forms when stagnant water or flooding restrict oxygen, slowing decomposition. Peat can exhibit varying properties such as organic matter content and hydraulic conductivity.

Soil fertility: Soil fertility refers to the soil's capacity to sustain consistent and high-quality agricultural plant growth. It involves supplying necessary plant nutrients and water in appropriate quantities over a sustained period. A fertile soil must also be free from toxic substances that can hinder plant growth.

Soil formation: Soil formation, or pedogenesis, is the process by which soil is created and shaped by the environment and history. Biogeochemical processes create and disrupt order in soils, leading to the formation of distinct layers called soil horizons. These horizons vary in color, structure, texture, and chemistry. The distribution of soil types is determined by different factors that influence the formation of soil.

Pedosphere: The pedosphere is the outermost layer of the Earth, made of soil and shaped by soil formation processes. It acts as the interface between the lithosphere, atmosphere, hydrosphere, and biosphere. Serving as the Earth's skin, it forms when there is a dynamic interaction between these components. Ultimately, the pedosphere is crucial for supporting terrestrial life on our planet.

Stratigraphy: Stratigraphy is the study of rock layers and their arrangement. It is used mainly for sedimentary and volcanic rocks. This field includes three subfields: lithostratigraphy, biostratigraphy, and chronostratigraphy.

Stratigraphic unit: A stratigraphic unit is a identifiable volume of rock defined by its unique characteristics, such as petrographic, lithologic, or paleontologic features. It represents a relative age range and is easily recognizable and mappable.

Fold (geology): A fold in geology refers to the bending or curving of originally planar surfaces, like sedimentary strata, during permanent deformation. Folds can range in size from microscopic crinkles to mountain-sized structures, occurring either as isolated folds or in periodic sets. Some folds are formed during the deposition of sediments and are known as synsedimentary folds.

Fault (geology): A fault is a fracture in rock that has caused significant displacement due to rock-mass movements. Plate tectonic forces create large faults, some of which form the boundaries between plates. Earthquakes are mainly caused by rapid movement on active faults, while some faults displace slowly without causing seismic activity.

Thrust fault: A thrust fault is a fracture in the Earth's crust that causes older rocks to move above younger rocks.

Plate tectonics: Plate tectonics is a scientific theory stating that Earth's lithosphere consists of large moving plates. This concept developed from the idea of continental drift in the early 20th century and gained acceptance after the validation of seafloor spreading in the mid-to-late 1960s.

Continent: A continent is a large geographical region conventionally identified, which may be a single landmass or part of a larger one. The number of continents can vary, with seven commonly recognized: Asia, Africa, North America, South America, Antarctica, Europe, and Australia. Some variations merge these regions, such as combining North and South America into America or Africa, Asia, and Europe into Afro-Eurasia.

Continental shelf: A continental shelf is a submerged portion of a continent, covered by shallow water called a shelf sea. These shelves were exposed during glacial periods when sea levels dropped. The shelf surrounding an island is called an insular shelf.

Abyssal plain: An abyssal plain is a flat and smooth underwater plain on the deep ocean floor. It is typically found between the foot of a continental rise and a mid-ocean ridge, at depths of 3,000 to 6,000 meters. Covering over 50% of the Earth's surface, abyssal plains are important geological features of oceanic basins and are among the least explored regions on our planet.

Mid-ocean ridge: A mid-ocean ridge (MOR) is a seafloor mountain system resulting from plate tectonics. It typically reaches a height of 2,000 meters above the ocean basin's deepest point. Located at divergent plate boundaries, it is where seafloor spreading occurs. The rate of seafloor spreading determines the ridge's shape and width.

Oceanic trench: Oceanic trenches are narrow, long depressions on the ocean floor, typically 50 to 100 kilometers wide and 3 to 4 km deep. They can stretch for thousands of kilometers and there are about 50,000 km of trenches worldwide. The deepest point is in the Mariana Trench's Challenger Deep, reaching 10,920 m below sea level. These trenches are mainly found in the Pacific Ocean, with some in the eastern Indian Ocean and a few other regions.

Lithosphere: The lithosphere is the rigid outer shell of a planet or moon. On Earth, it consists of the crust and the top part of the upper mantle. It behaves elastically and is distinguished by its chemistry and mineralogy.

Crust (geology): The crust is the outer solid layer of rocky planets and satellites. It differs from the mantle in chemical composition and can also be distinguished by its phase in icy satellites.

Volcano: A volcano is a rupture in the Earth's crust, through which hot lava, ash, and gases escape from a magma chamber below the surface.

Geyser: A geyser is a spring that sporadically shoots out water and steam due to unique hydrogeological conditions found in a few locations on Earth. It is a rare phenomenon.

Hotspot (geology): Hotspots are volcanic areas fueled by abnormally hot mantle beneath the Earth's surface. They can be found independently of tectonic plate boundaries and are responsible for volcano chains as plates shift. Notable examples include Hawaii, Iceland, and Yellowstone.

Lava: Lava is molten rock expelled from a planet or moon's interior onto its surface. It can erupt from a volcano or through a crust fracture, both on land or underwater, at temperatures ranging from 800 to 1,200 °C. The cooled volcanic rock is also referred to as lava.

Geomorphology: Geomorphology is the study of how landforms are shaped by various processes. Geomorphologists examine landform history and predict changes using field observations, experiments, and modeling. They work in fields like geography, geology, archaeology, and engineering.

Erosion: Erosion is the process of removing and transporting soil, rock, or dissolved material from one place to another on Earth's crust. It is different from weathering as it involves movement. Physical erosion refers to the removal of rock or soil as sediment, while chemical erosion dissolves soil or rock. Eroded material can be transported over short distances or for long distances, ranging from millimeters to thousands of kilometers.

Avalanche: An avalanche is a fast-moving snow descent on a hill or mountain slope.

Landslide: Landslides, or landslips, refer to various types of ground movements like rockfalls, mudflows, and debris flows. They can happen in different environments, ranging from mountains to underwater areas. Landslides occur on steep or gentle slopes and can cause significant damage.

Weathering: Weathering is the natural decay of rocks, soils, and minerals caused by water, air, sunlight, and living organisms. It happens where these materials are found, unlike erosion which involves the movement of rocks and minerals by water, wind, and other forces.

Landform: A landform is a physical feature on the Earth's surface. It includes hills, mountains, canyons, valleys, and shoreline features like bays and peninsulas. Landforms form the landscape known as topography and can be natural or man-made. They also include submerged features such as mid-ocean ridges, volcanoes, and ocean basins.

Land: Land, also called dry land or ground, is the solid surface of Earth not covered by water. It represents 29.2% of the planet's surface and includes continents and islands. Covered mostly by regolith, a layer of rock, soil, and minerals, land influences Earth's climate through its involvement in the carbon, nitrogen, and water cycles. A third is forested, another third is used for agriculture, and a tenth is made up of permanent snow and glaciers. The rest comprises desert, savannah, and prairie.

Hill: A hill is an elevated landform with a distinct summit, extending above the surrounding terrain. It is commonly applied to peaks that are higher than the land around them but not as prominent as mountains.

Valley: A valley is a long, low area between hills or mountains, usually with a river or stream flowing through it. Valleys are predominantly formed by the erosion caused by rivers or streams over a long period, while some are the result of glacial ice erosion, often found in high mountains or polar regions.

Plateau: A plateau is a flat, raised area with steep sides, found in highlands. It can be formed by volcanic activity, lava flow, or water and glacier erosion. Plateaus are classified based on their surroundings as intermontane, piedmont, or continental. They can have small or wide flat tops.

Plain: A plain is a flat expanse of land, typically without trees, that remains relatively level in elevation. They can be found in valleys, at the base of mountains, along coastlines, and on plateaus. Plains are found on all continents, covering over one-third of the world's land area. They are significant for agriculture and support various types of biomes.

Impact crater: An impact crater is a circular depression on a solid object caused by the high-speed collision of a smaller object. Unlike volcanic craters, impact craters have elevated rims and lower floors. Lunar impact craters vary in size from tiny craters on lunar rocks to large, multi-ringed basins. Meteor Crater on Earth is a notable example of a small impact crater.

Aeolian processes: Aeolian processes refer to wind activity in geology and weather, shaping the Earth's surface through erosion, transport, and deposition of materials. They are significant in arid environments like deserts, where sparse vegetation, lack of soil moisture, and abundant loose sediments create ideal conditions for wind to shape the land. While water is a more powerful eroding force, aeolian processes play a crucial role in these dry regions.

Dune: A dune is a landform made of wind or water-driven sand, often in the shape of a mound or ridge. Areas with dunes are called dune systems or complexes, while larger ones are called dune fields. Flat regions covered with sand and little vegetation are known as ergs or sand seas. Dunes come in various shapes and sizes, with longer sides facing the wind and shorter slip faces on the opposite side. The space between dunes is called a dune slack.

Artificial island: An artificial island is a land constructed by humans rather than formed naturally. It can be created by expanding existing islets, constructing on reefs, or combining multiple islets. The size of artificial islands can vary, from small reclaimed areas for a single structure to entire communities. The concept is not new and dates back to the Neolithic era. Early artificial islands included floating structures or wooden/megalithic structures in shallow waters.

Land reclamation: Land reclamation, also known as reclamation or land fill, is the process of transforming underwater or wet areas like oceans, seas, riverbeds, or lake beds into usable land. This newly created land is referred to as reclamation ground, reclaimed land, or land fill.

Polder: A polder is a low-lying area enclosed by dikes that forms an artificial landmass. It can be created by reclaiming land from bodies of water, separating flood plains from rivers or seas, or draining marshes. In Germany, the term koogs is used interchangeably.

Cave: A cave or cavern is a natural void large enough for humans to enter, formed by the weathering of rock. It can include smaller openings like sea caves and rock shelters. Caves that extend deeper underground than their openings are called endogene caves.

Karst: Karst is a unique landscape shaped by the erosion of limestone, dolomite, and gypsum, resulting in sinkholes and underground caves. It includes features like poljes and drainage systems. Additionally, under suitable conditions, less susceptible rocks like quartzite can also be found.

Alluvial fan: An alluvial fan is a sediment accumulation that spreads out from its source, typically a narrow canyon. They are found in mountainous areas with dry or wet climates, ranging in size from less than 1 square kilometer to 20,000 square kilometers.

Canyon: A canyon is a deep cleft between cliffs resulting from the erosive activity of a river over time. It forms when a river cuts through layers of rock, wearing them away as sediments are carried downstream. Canyons occur when the river's starting point and endpoint have different elevations, especially in areas with both soft and hard rock layers.

Floodplain: A floodplain is a low-lying area next to a river that floods during high discharge. It extends from the river banks to the enclosing valley and is composed of soils like clay, silt, sand, and gravel deposited during floods.

Meander: A meander is a sinuous curve in a watercourse formed by erosion of its outer bank and deposition on its inner bank. This creates a winding channel that migrates across a floodplain due to sedimentation and erosion.

Rapids: Rapids are fast-flowing and turbulent sections of a river caused by a steep river bed gradient. Four factors required for rapids include strong water flow, a sharp incline, narrow passages, and obstacles.

Waterfall: A waterfall is when water flows over a vertical drop or steep drops in a river or stream. It can also happen when meltwater falls off the edge of a tabular iceberg or ice shelf.

Glacial landform: Glacial landforms are formations shaped by glaciers, predominantly during the Quaternary glaciations. Prominent occurrences exist in Fennoscandia and the southern Andes, while rare and ancient fossil glacial landforms are found in places like the Sahara.

Fjord: A fjord is a narrow sea inlet with steep sides, formed by a glacier. They are found in Antarctica, the Arctic, and northern/southern hemispheres. Norway has about 1,200 fjords, making its coastline around 29,000 km long.

Moraine: A moraine is debris carried by a glacier or ice sheet and found in glaciated areas. It can include particles of various sizes, from boulders to gravel and sand, mixed with fine clay called glacial flour. Lateral moraines form at the side of the ice flow, while terminal moraines mark the furthest extent of the glacier. Other types of moraines include ground and medial moraines.

Mountain: A mountain is a raised part of the Earth's crust with steep sides, often showing exposed bedrock. It is usually higher than a hill, rises at least 300 metres above surrounding land, and has a limited summit area. Mountains can be found either as isolated summits or within mountain ranges.

Mountain range: A mountain range is a series of mountains or hills connected by high ground. They are typically formed by plate tectonics and can be part of a mountain system with similar form, structure, and alignment. Mountain ranges are not exclusive to Earth and are likely present on most terrestrial planets in the Solar System.

Archipelago: An archipelago refers to a group of islands or a cluster of scattered islands in a sea. It can also be known as an island chain.

Bay: A bay is a coastal body of water connecting to a larger water body, like an ocean or lake. It can be large and called a gulf or a sea. A cove is a small circular bay with a narrow entrance. Fjords are elongated bays formed by glaciers. Embayment is used for related features like extinct bays or freshwater environments.

Beach: A beach is a landform consisting of loose particles, made from rocks or organic sources. The particles vary in density and structure due to wave action and weather, resulting in different textures, colors, and layers of material.

Coast: A coast is the area where land meets the ocean, forming a boundary between land and sea. It is influenced by the surrounding landscape and water-induced erosion. Coasts are important for ecosystems and biodiversity, including wetlands, saltmarshes, mangroves, and rocky shores. They also provide habitat for various aquatic species. Additionally, coral reefs can be found in certain coastal areas. The Earth has approximately 620,000 kilometers of coastline.

Lagoon: A lagoon is a shallow water body separated from a larger body of water by a narrow landform. It can be classified as a coastal or atoll lagoon and occurs on mixed-sand and gravel coastlines. There is an overlap between lagoons and estuaries. They are common coastal features worldwide.

Peninsula: A peninsula is a landform surrounded by water on most of its borders. It can also be defined as land bordered by water on three sides. Peninsulas exist on all continents and can vary in size. The Arabian Peninsula is the largest in the world. They form due to various causes.

Island: An island is a piece of land surrounded by water. Small islands are called islets, and those in rivers or lakes may be called eyots or aits. Other names for small islands off the coast include holms, cays, skerries, and keys. Sedimentary islands in the Ganges Delta are called chars. A collection of islands, such as the Philippines, is known as an archipelago.

Atoll: An atoll is a ring-shaped island with a coral rim enclosing a lagoon. It forms in warm tropical or subtropical waters where coral can grow. The Pacific Ocean is home to most of the world's 440 atolls, which may also include coral islands or cays.

Seamount: A seamount is a large underwater landform that does not reach the ocean surface, formed from extinct volcanoes. They rise abruptly from the seafloor, reaching heights of 1,000-4,000m. These conical features are found hundreds to thousands of meters below the surface, making them part of the deep sea. Some seamounts eventually erode to create flat surfaces called guyots or tablemounts.

Cliff: A cliff is a vertical or nearly vertical area of rock, formed by weathering and erosion, as a result of gravity. They are found in various locations including coasts, mountains, escarpments, and rivers. Cliffs are typically made of resistant rocks like sandstone, limestone, chalk, dolomite, granite, and basalt.

Mesa: A mesa is an isolated flat-topped hill with steep sides, standing prominently above a plain. It is formed by flat-lying sedimentary rocks topped by a more resistant layer, creating a caprock. This caprock can consist of sandstone, limestone, lava flows, or eroded duricrust. It is different from a plateau, as mesas specifically involve horizontal layers of bedrock, while flat-topped plateaus are called tablelands.

Cryosphere: The cryosphere refers to all areas on Earth's surface where water is frozen, such as sea ice, glaciers, and snow cover. It plays a crucial role in the global climate system and impacts various aspects like energy flux, precipitation, and oceanic circulation. It is closely related to the hydrosphere, contributing to the overall functioning of Earth's climate.

Flood: A flood is the overflow of water onto usually dry land. It can also refer to the incoming tide. Floods are studied in hydrology and are significant concerns for agriculture, civil engineering, and public health. Man-made environmental changes, like deforestation and removal of wetlands, increase the frequency and intensity of floods. Climate change exacerbates flooding through increased rainfall and extreme weather events, leading to more severe and frequent floods with higher risk.

Hydrosphere: The hydrosphere refers to all water found on, under, and above a planet's surface. Earth's hydrosphere is over 4 billion years old and constantly changing due to seafloor spreading and continental drift.

Oceanography: Oceanography, also known as oceanology, is the scientific study of the oceans. It covers various topics such as ecosystem dynamics, ocean currents, plate tectonics, and chemical substances within the ocean. Oceanographers utilize multiple disciplines such as astronomy, biology, chemistry, geography, geology, hydrology, meteorology, and physics to gain knowledge about the world ocean. Additionally, paleoceanography examines the past history of the oceans. This field encompasses the study of marine geology, physics, chemistry, and biology.

Coral reef: A coral reef is an underwater ecosystem made up of colonies of coral polyps joined by calcium carbonate. It is primarily formed from stony corals that gather in groups.

Coral reef fish: Coral reef fish thrive in coral reef ecosystems, exhibiting vibrant colors and diversity. They inhabit small areas within healthy reefs, displaying adaptations for survival and often remaining hidden or camouflaged.

Deep-sea fish: Deep-sea fish inhabit the dark depths below the sunlit surface waters of the ocean. The lanternfish is the most prevalent among them, with other notable species including flashlight fish, cookiecutter shark, bristlemouths, anglerfish, viperfish, and certain eelpout species.

Ocean: The ocean is a vast body of salt water covering around 70.8% of the Earth's surface. It is divided into five areas: Pacific, Atlantic, Indian, Antarctic/Southern, and Arctic. The ocean contains 97% of Earth's water and is crucial for life on our planet. It greatly influences climate, weather patterns, the carbon cycle, and acts as a massive heat reservoir.

Ocean current: An ocean current is a sustained, flowing movement of seawater caused by various factors such as wind, temperature and salinity differences, and the Coriolis effect. These currents are primarily horizontal and influenced by depth, shoreline, and interactions with other currents.

Thermohaline circulation: Thermohaline circulation (THC) is a global oceanic movement driven by temperature and salt content variations, which determine water density. Surface currents carry cool water from the equator to high latitudes where it sinks and flows into ocean basins. This circulation mixes waters between basins, making Earth's oceans a connected global system. THC plays a crucial role in transporting energy and mass worldwide, thus significantly impacting Earth's climate.

Sea: A sea is a large body of salty water. It can refer to specific seas or the ocean, which is a wider body of seawater. Specific seas can be marginal seas, sections of the ocean, or large, nearly landlocked bodies of water.

Seabed: The seabed refers to the ocean floor, encompassing the entirety of the ocean. It includes all the floors and is commonly known as 'seabeds.'

Sea level: Sea level is an average surface level of Earth's coastal bodies of water used as a reference for measuring elevation. It is a global vertical datum that serves as a standard for cartography, marine navigation, aviation, and atmospheric pressure measurements. A typical mean sea-level standard is the midpoint between a mean low and mean high tide at a specific location.

Tide: Tides are the result of gravitational forces between the Moon, Earth, and their orbital interaction, leading to the regular rise and fall of sea levels.

Tsunami: A tsunami is a series of waves caused by the displacement of a large volume of water, often by natural disasters like earthquakes or volcanic eruptions. Unlike regular ocean waves or tides, tsunamis result from significant water movement caused by major events.

Glacier: A glacier is a dense, moving body of ice that forms when snow accumulation exceeds melting over many years. It creates distinct features such as crevasses and moraines, as it flows and shapes the land. Glaciers only form on land and are different from sea ice and lake ice.

Iceberg: An iceberg is a large chunk of freshwater ice that has broken off from a glacier or ice shelf and is floating in saltwater. It poses a serious risk to maritime navigation. Only a small part of an iceberg is visible above the water, leading to the saying "tip of the iceberg" to represent a small visible portion of a larger unseen issue. Smaller pieces of floating ice are called "growlers" or "bergy bits".

Ice sheet: An ice sheet, or continental glacier, is a vast mass of glacial ice covering more than 50,000 km². The only existing ice sheets are the Antarctic ice sheet and the Greenland ice sheet. Ice sheets are larger than ice shelves or alpine glaciers. Smaller ice masses covering less than 50,000 km² are called ice caps, which typically feed surrounding glaciers.

Ice shelf: An ice shelf is a large floating platform of ice that forms where glaciers or ice sheets flow into the ocean. They are found in Antarctica and the Arctic, with thickness ranging from 100 to 1,000 meters. The grounding line separates the floating ice shelf from the anchor ice. The Ross and Filchner-Ronne Ice Shelves are the world's largest. Ice shelves breaking off can create icebergs, a process known as ice calving.

Permafrost: Permafrost is frozen soil or underwater sediment that stays below freezing for at least two years. It can be as old as 700,000 years and can reach depths greater than 1,500 meters. Permafrost can be found in narrow mountain summits or vast Arctic regions. It is usually located below a layer of soil that freezes and thaws with the seasons, excluding areas under glaciers and ice sheets.

Sea ice: Sea ice is frozen seawater that floats on the ocean's surface. It covers 7% of the Earth's surface and 12% of the world's oceans. It is mainly found in the polar ice packs of the Arctic and Antarctic regions. Sea ice undergoes yearly cycling and plays a crucial role in the Arctic ecosystem. It is highly dynamic and varies in types and features due to winds, currents, and temperature changes. Sea ice is distinct from icebergs, which are chunks of ice from glaciers or ice shelves. In some areas, sea ice expanses may include icebergs.

Hydrology: Hydrology is the science of studying water movement and management on Earth and other planets. It involves analyzing data to solve water-related issues such as environmental preservation, natural disasters, and water management. Hydrologists use various scientific techniques to collect and analyze data.

Groundwater: Groundwater is the water beneath Earth's surface in rock and soil pores. It accounts for 30% of accessible freshwater. Aquifers are rock or deposits that contain usable amounts of water. The water table is the depth at which pore spaces are saturated. Groundwater can be naturally discharged or form wetlands. It is extracted for human use and studied in hydrogeology.

Spring (hydrology): A spring is a natural point where groundwater emerges and becomes surface water. It is vital for humans, particularly in arid regions with low annual rainfall, as a source of fresh water. Springs are an important component of the water cycle and the hydrosphere.

Hydrography: Hydrography is the scientific study of oceans, seas, lakes, and rivers. It involves measuring and describing their physical features and predicting changes over time. The main goal is to ensure safe navigation and support various marine activities, including economic development, defense, research, and environmental protection.

Lake: A lake is a large body of water on the earth's surface, separate from the ocean. It is surrounded by land and serves as a storage for freshwater, forming part of the Earth's water cycle. While most lakes are freshwater and account for the majority of the world's surface freshwater, some are salt lakes with higher salinity than seawater. Lakes vary in size and volume.

Dry lake: A dry lake, or playa, is a basin that once had water but now dries up due to evaporation. If the lake bed has alkaline deposits, it becomes an alkali flat, and if it has salt deposits, it becomes a salt flat.

Pond: A pond is a small body of water formed by pooling inside a depression. It is smaller than a lake and can be naturally or artificially created. Ponds can have emergent vegetation and are categorized into four zones. They can vary in size and depth throughout the year and can be freshwater or brackish. Saltwater ponds connected to the sea are considered part of the marine environment and do not support fresh or brackish water-based organisms.

Limnology: Limnology is the study of inland water systems, encompassing their biological, chemical, physical, and geological properties. It investigates various natural and man-made bodies of water like lakes, rivers, wetlands, and groundwater. These water systems are classified as either running or standing.

River: A river is a natural freshwater stream that flows on the earth's land surface towards another larger body of water. It can also end by flowing into the ground or drying up. Small rivers have different names depending on the region. The term river doesn't have an official definition, and its size can vary.

Drainage basin: A drainage basin is an area where all surface water collects and flows to a single point or into another body of water. It is separated from adjacent basins by elevated features called the drainage divide. Basins can be made up of smaller basins that merge at river confluences, forming a hierarchical pattern.

Estuary: An estuary is a coastal body of brackish water, formed when rivers or streams flow into it and connect to the open sea. It serves as a transition zone between freshwater and marine environments, providing high levels of nutrients in both water and sediment. Estuaries are influenced by tides, waves, saline water, freshwater flows, and sediment, making them highly productive natural habitats.

River delta: A river delta is a triangular landform created when sediment carried by a river is deposited in slower-moving or stagnant water. It occurs at the river mouth, where it enters an ocean, sea, estuary, lake, reservoir, or another river unable to carry away the sediment. The delta resembles the Greek letter Δ. Its size and shape are determined by the balance between sediment supply and redistribution, sequestration, and export processes in the receiving basin. The receiving basin's size, geometry, and location also influence delta evolution.

Water cycle: The water cycle is the continuous movement of water on, above, and below the Earth's surface. It involves various processes like evaporation, condensation, precipitation, and runoff. The water can be found in different forms – liquid, solid, and vapor. The ocean is a major source of evaporation in the water cycle.

Physics: Physics is the study of matter, its motion, behavior, and the concepts of energy and force. It aims to comprehend the behavior of the universe. Physicists are the scientists who specialize in this field.

Energy: Energy is a fundamental property in physics that can be transferred to a system and is observed through work, heat, and light. It cannot be created or destroyed, only converted. The joule (J) is the unit used to measure energy in the SI system.

Conservation of energy: The conservation of energy states that the total energy in an isolated system remains constant over time. Energy can be transformed or transferred between different forms, but it cannot be created or destroyed. In a closed system, energy can only change through entering or leaving the system. An example is the conversion of chemical energy into kinetic energy during an explosion. Adding up all released forms of energy will equal the decrease in chemical energy during the explosion.

Matter: Matter refers to substances that have mass and occupy space. It consists of atoms and particles that possess both rest mass and volume. Matter encompasses solids, liquids, and gases like water. It does not include massless particles, energy phenomena, or waves such as light or heat. Additionally, matter can exist in other states like plasma, Bose–Einstein condensates, fermionic condensates, and quark–gluon plasma.

Vacuum: A vacuum is an empty space devoid of matter. It is derived from the Latin word vacuus, meaning "vacant" or "void". Physicists refer to a perfect vacuum as a space without any matter or pressure, while a partial vacuum is an imperfect vacuum with reduced pressure. In engineering and applied physics, vacuum refers to a space with lower pressure than the atmosphere. The term in vacuo is used to describe an object surrounded by a vacuum.

Gravity: Gravity is a fundamental interaction that causes attraction between objects with mass. It is the weakest of the four fundamental interactions and has no significant influence on subatomic particles. However, gravity plays a crucial role in determining the motion of macroscopic objects such as planets, stars, galaxies, and light.

Electromagnetism: Electromagnetism is a fundamental force in nature, involving interactions between charged particles through electromagnetic fields. It combines electrostatics and magnetism, resulting in attractive or repulsive forces between charged particles. This force dominates atomic and molecular interactions. Electromagnetism also creates electromagnetic fields that accelerate particles via the Lorentz force. At high energy, electromagnetism is unified with the weak force to form the electroweak force.

Strong interaction: The strong interaction, also known as the strong force or strong nuclear force, is a fundamental interaction in nuclear and particle physics. It confines quarks into hadron particles like protons and neutrons, and binds these particles to form atomic nuclei. It is essential for the stability and structure of matter.

Weak interaction: The weak interaction, a fundamental force in nuclear and particle physics, is responsible for radioactive decay and nuclear reactions. It is one of the four known fundamental interactions, along with electromagnetism, the strong interaction, and gravity. Its behavior is described by electroweak theory, also known as quantum flavor dynamics.

Spacetime: Spacetime is a mathematical model combining space and time into a four-dimensional continuum. It helps visualize relativistic effects, like how events appear to different observers.

Space: Space is a three-dimensional continuum of positions and directions that is crucial to understanding the physical universe. It is usually seen as a three-dimensional concept in classical physics, but modern physicists view it as part of a four-dimensional spacetime framework. Philosophical debate exists on whether space is an entity, a relationship between entities, or a conceptual framework.

Time: Time is the ongoing sequence of events that occurs in a forward direction from the past to the future. It is used to measure durations, intervals, and rates of change. Time is commonly considered a fourth dimension alongside three spatial dimensions.

Mass–energy equivalence: Mass-energy equivalence refers to the relationship between mass and energy in a system's rest frame, where they differ by a multiplicative constant. Albert Einstein's formula describes this principle. In a moving reference frame, the system's relativistic energy and relativistic mass follow the same formula.

Radiation: Radiation is the emission or transmission of energy through waves or particles. It can be categorized into electromagnetic radiation (e.g. radio waves, x-rays) and particle radiation (e.g. alpha and beta particles). Additionally, acoustic radiation includes ultrasound and seismic waves, while gravitational radiation refers to gravitational waves.

Ionizing radiation: Ionizing radiation is a form of energy that can detach electrons from atoms or molecules. It includes subatomic particles and high-energy electromagnetic waves. Some particles can travel close to the speed of light.

Field (physics): In physics, a field is a physical quantity that has a value for each point in space and time. It can be represented as a scalar, vector, or tensor. Field theories are used to describe how the values of fields change in space and time. Examples include temperature and wind speed maps, which are scalar and vector fields respectively. The electric field is another example of a field, which can be described as a rank-1 tensor field. Electrodynamics involves the interaction of two vector fields or a single rank-2 tensor field. Fields are fundamental in understanding the dynamics of physical phenomena.

Flux: Flux refers to the passage or movement of something through a surface or substance. In applied mathematics and vector calculus, flux has various applications in physics. It can be a vector quantity representing the flow's magnitude and direction in transport phenomena. In vector calculus, it is a scalar quantity obtained by integrating the perpendicular component of a vector field over a surface.

Gauge theory: Gauge theory is a type of field theory in physics. It involves a Lagrangian that remains unchanged under certain local transformations, meaning the dynamics of the system remain consistent. The Lagrangian is invariant.

Symmetry (physics): Symmetry in physics refers to a feature of a physical system that remains unaltered under certain transformations. This preservation could be either physical or mathematical in nature.

Theoretical physics: Theoretical physics is a field of physics that uses mathematics and abstractions to understand, clarify, and forecast natural phenomena. It differs from experimental physics, which relies on experiments to investigate these phenomena.

Speed of light: The speed of light (c) is a universal physical constant of 299,792,458 m/s in vacuum. In line with the special theory of relativity, it serves as the maximum speed at which conventional matter or energy can move through space.

Atom: An atom is the basic particle of the chemical elements, composed of a nucleus with protons and often neutrons, surrounded by electrons. The number of protons in an atom determines its identity as a particular element. Atoms with the same number of protons but different numbers of neutrons are called isotopes.

Atomic physics: Atomic physics is a branch of physics that investigates the structure of atoms, particularly the arrangement of electrons around the nucleus, and the processes that cause this arrangement to change. It explores the interaction between atoms and includes the study of ions and neutral atoms.

Atomic theory: Atomic theory is the scientific belief that matter is made up of tiny particles called atoms. It gained credibility in the 18th and 19th centuries as it explained gas behavior and chemical reactions. By the late 19th century, atomic theory was widely accepted by the scientific community.

Atomic nucleus: The atomic nucleus is a small, dense region at the center of an atom, composed of protons and neutrons. It was discovered in 1911 and further understood after the discovery of the neutron in 1932. The nucleus carries most of the atom's mass and is bound together by the nuclear force. It is surrounded by negatively charged electrons held together by electrostatic force.

Isotope: Isotopes are different versions of the same element with the same atomic number but different numbers of neutrons. They share similar chemical properties but have different atomic masses and physical properties.

Nuclide: A nuclide refers to atoms distinguished by their proton and neutron count, denoted as Z and N respectively. It also includes their nuclear energy state.

Atomic orbital: An atomic orbital is a mathematical function that describes the location and wave-like behavior of an electron in an atom. It can be used to calculate the probability of finding the electron in different regions around the atom's nucleus. The term may also refer to the physical space where the electron is predicted to be present based on the orbital's mathematical form.

Binding energy: Binding energy is the minimum energy needed to separate particles or dismantle a particle system. It is primarily used in condensed matter physics, atomic physics, and chemistry, while nuclear physics refers to it as separation energy.

Bohr model: The Bohr model, proposed by Niels Bohr and Ernest Rutherford in 1913, is an atomic model that depicts a dense nucleus orbited by electrons, similar to the Solar System. The model emphasizes the quantized energy levels of electrons and the electrostatic force as the binding force, instead of gravity.

Electron configuration: Electron configuration is the arrangement of electrons in an atom's orbitals. It determines an atom's chemical properties. For instance, neon's electron configuration is 1s2 2s2 2p6, showing the occupation of specific subshells.

Electron shell: An electron shell is an orbit followed by electrons around an atom's nucleus. They are labeled numerically or alphabetically, representing principal quantum numbers or X-ray notation. Each row on the periodic table signifies an electron shell.

Ion: An ion is an atom or molecule that carries an electrical charge. In an ion, the number of electrons is not equal to the number of protons, causing a net charge. Protons are considered positive, while electrons are considered negative.

Energy level: Energy levels are specific values of energy that a bound quantum system or particle can have. This is in contrast to classical particles that can have any amount of energy. Energy levels are commonly used to describe the electrons in atoms, ions, or molecules, but can also refer to nuclei or vibrational and rotational energy in molecules. The discrete energy levels in a system lead to a quantized energy spectrum.

Geiger–Marsden experiments: The Geiger–Marsden experiments were conducted between 1908 and 1913 by Hans Geiger and Ernest Marsden under the supervision of Ernest Rutherford at the University of Manchester. These experiments revealed that every atom has a concentrated nucleus housing most of its mass and positive charge. Scientists made this discovery by observing the scattering of alpha particles when they struck a thin metal foil.

Molecular orbital: A molecular orbital is a mathematical function that describes the location and wave-like behavior of an electron in a molecule. It helps calculate properties like the probability of finding an electron in a specific region. Atomic and molecular orbitals were introduced in 1932 to represent one-electron wave functions. They describe the region in space where a function has a significant amplitude.

Molecular orbital theory: Molecular orbital theory is a quantum mechanics approach to understand the electronic structure of molecules. It was proposed in the early 20th century.

Optics: Optics is the study of light and its interaction with matter. It encompasses visible, ultraviolet, and infrared light, as well as other forms of electromagnetic radiation like X-rays and microwaves. This field also investigates the construction of devices that utilize or detect light.

Focus (optics): A focus, or image point, in optics refers to the convergence of light rays originating from an object. It is not a precise point but has a blur circle due to imperfections in the imaging optics called aberrations. The smallest possible blur circle is known as the Airy disc, resulting from diffraction caused by the aperture of the optical system. Aberrations worsen with larger aperture diameters, while the Airy circle is smallest for larger apertures.

Focal length: The focal length measures how light converges or diverges in an optical system. A positive focal length shows convergence, while a negative focal length indicates divergence. A shorter focal length bends light more sharply, bringing it to a focus quicker. In the case of a thin lens in air, a positive focal length is the distance for collimated rays to focus, while a negative focal length shows where a point source must be located to form a collimated beam. For general optical systems, the focal length is the inverse of the system's optical power.

Polarization (waves): Polarization is a property of transverse waves that describes the orientation of their oscillations. Unlike longitudinal waves, transverse waves exhibit polarization. Examples of polarized waves include light, radio waves, and transverse sound waves in solids. The direction of oscillation can be vertical, horizontal, or any angle perpendicular to the wave's motion. Longitudinal waves, like sound in liquids or gases, do not show polarization. Gravitational waves also exhibit polarization.

Reflection (physics): Reflection (physics) refers to the redirecting of a wavefront at the boundary between two mediums, causing it to return to its origin. This phenomenon is observed in light, sound, and water waves. According to the law of reflection, the angle of incidence on the surface equals the angle of reflection for specular reflection.

Refraction: Refraction is the redirection of a wave as it moves between different mediums, caused by changes in speed or medium. It is most commonly observed with light, but also occurs with sound and water waves. The extent of refraction depends on the change in wave speed and the initial direction of propagation relative to the speed change.

Refractive index: The term 'Refractive index' refers to a dimensionless number in optics that indicates the light bending ability of an optical medium.

Transparency and translucency: Transparency refers to the property of allowing light to pass through a material without scattering. It follows Snell's law and has a uniform index of refraction, appearing clear with a single color or a spectrum of colors. Translucency also allows light to pass through but can scatter it at interfaces or internally due to different indices of refraction. Opacity is the opposite of translucency. These properties, along with regular and diffuse reflection and transmission of light, are categorized as aspects of visual appearance under the concept of cesia.

Color: Color or colour is the perception of light based on the electromagnetic spectrum. It is not an inherent property of matter, but rather linked to an object's light absorption, reflection, emission, and interference. Humans perceive colors in the visible light spectrum using three types of cone cells. However, other animals may have different numbers and types of cone cells, allowing them to perceive different wavelengths, like bees distinguishing ultraviolet. Animal color perception is determined by the brain's processing of light wavelengths detected by their cone cell types.

CMYK color model: The CMYK color model is used in color printing and process, referring to the four ink plates: cyan, magenta, yellow, and key (black). It is a subtractive model derived from the CMY color model.

Color theory: Color theory is a practical guide for mixing colors and understanding their visual effects. It dates back to ancient times, with Aristotle and Claudius Ptolemy exploring color mixing. Al-Kindi and Ibn al-Haytham delved into the influence of light on color. Ibn Sina, Nasir al-Din al-Tusi, and Robert Grosseteste discovered multiple paths to create different colors. Modern approaches can be seen in the works of Alberti and da Vinci. The formalization of color theory began in the 18th century, with debates over Newton's theory of color. It has evolved as an independent artistic tradition, with limited reference to colorimetry and vision science.

Primary color: Primary colors are colorants or colored lights that can be mixed to create a wide range of colors. They are used in electronic displays, color printing, and paintings. The perception of colors can be predicted using a mixing model based on how light interacts with physical media and the retina.

Gloss (optics): Gloss in optics refers to how well a surface reflects light in a mirror-like direction. It is an important parameter for describing the visual appearance of an object. Gloss is part of a larger concept called cesia, which encompasses various aspects of visual appearance. Factors affecting gloss include material refractive index, angle of incident light, and surface topography.

RGB color model: The RGB color model combines red, green, and blue light to create a wide range of colors. Its name is derived from the initials of these primary colors.

Black: Black is an achromatic color that occurs when visible light is completely absorbed. Often used symbolically to represent darkness, it has been associated with oppositions like good and evil, as well as night and day. Since the Middle Ages, black has been a symbolic color of seriousness and power, commonly worn by judges and magistrates.

Blue: Blue is a primary color in both the RYB and RGB color models. It falls between violet and cyan on the visible light spectrum. It describes colors perceived by humans with wavelengths between approximately 450 and 495 nanometers. Blues often contain some green or violet. The daytime sky and deep sea appear blue due to Rayleigh scattering, while blue eyes are explained by the Tyndall effect. Distant objects appear bluer due to aerial perspective.

Brown: Brown is a composite color and a darker shade of orange. It is created by combining orange and black in the CMYK color model, and red and green in the RGB color model.

Green: Green is a color between cyan and yellow, with a wavelength of 495-570 nm. It is created by combining yellow and cyan in subtractive color systems and is one of the additive primary colors in the RGB model. Its main natural source is chlorophyll, aiding photosynthesis in plants. Many creatures camouflage themselves in green environments. Green minerals, such as emerald, contain chromium and exhibit this color.

Grey: Grey or gray is an intermediate, neutral color between black and white, composed of both colors. It represents cloud-covered skies, ash, and lead.

Orange (colour): Orange is a color that lies between yellow and red on the light spectrum. It appears when our eyes perceive light with a wavelength of around 585 to 620 nanometers. In color theory, it is created by mixing yellow and red pigments, while in the RGB model, it is considered a tertiary color. Its name is derived from the fruit called orange.

Pink: Pink is a pale tint of red, originally named in the late 17th century. It is associated with charm, sensitivity, femininity, and romance. Pink and white symbolize innocence, while pink and black represent eroticism. In modern times, pink is a symbol of femininity, but in the 1920s, it was considered masculine.

Purple: Purple is a color resembling violet light. It is a secondary color made by combining red and blue pigments. The CMYK color model for printing uses magenta pigment with cyan or black to create purple. On computer and TV screens, purple is achieved by mixing red and blue light to create violet-like hues.

Red: Red is a primary color in RGB and a secondary color in CMYK. It has a wavelength of 625-740 nm and is opposite violet. Red can vary from yellowish scarlet to bluish-red crimson and shades from pale red to dark burgundy.

White: White is the lightest color, opposite of black. It is the color of snow, chalk, and milk. White objects reflect and scatter all visible light. On screens, it is made by mixing red, blue, and green light. White is created with titanium dioxide pigments.

Yellow: Yellow is a color found between green and orange, with a dominant wavelength of 575-585 nm. It is used in painting and color printing as a primary color. In the RGB color model, yellow is a secondary color made by combining red and green. Carotenoids give yellow hues to various items like autumn leaves, corn, canaries, and lemons. They provide protection to plants and absorb light energy. Sunlight appears slightly yellow near the horizon due to atmospheric scattering.

Condensed matter physics: Condensed matter physics studies the physical properties of matter, particularly in solid and liquid phases, by examining electromagnetic forces between atoms and electrons. It explores condensed phases with strong interactions, including superconducting, ferromagnetic, antiferromagnetic, and Bose–Einstein condensate phases. Scientists use experiments and mathematical models based on quantum mechanics, electromagnetism, and statistical mechanics to understand these phases and predict the behavior of large groups of atoms.

Solid-state physics: Solid-state physics is the study of solids using methods like quantum mechanics and crystallography. It explores how the atomic-scale properties of solid materials determine their larger-scale properties. It is the largest branch of condensed matter physics and forms the theoretical basis of materials science. It has practical applications in the technology of transistors and semiconductors.

Crystallography: Crystallography is the study of how atoms are arranged in crystalline solids. It is a crucial field in materials science and solid-state physics. The term comes from Greek words meaning "crystal" and "writing". In 2012, the United Nations designated 2014 as the International Year of Crystallography, acknowledging its importance.

Crystal: A crystal is a solid material with a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. It is identifiable by its geometrical shape with specific orientations. Crystallography is the scientific study of crystals and their formation process is called crystallization or solidification.

Crystallization: Crystallization is a process where atoms or molecules arrange into a crystal structure. It occurs through methods like solution precipitation, freezing, or direct gas deposition. The resulting crystal's attributes depend on factors like temperature, air pressure, and evaporation time for liquid crystals.

State of matter: States of matter are distinct forms in which matter can exist. The four observable states in everyday life are solid, liquid, gas, and plasma. Other intermediate and exotic states, like liquid crystal and Bose-Einstein condensates, exist under extreme conditions. Neutron-degenerate matter and quark-gluon plasma are also examples of unique states. For more details, refer to the Wikipedia article List of states of matter.

Solid: A solid is a fundamental state of matter, along with liquid, gas, and plasma. It is characterized by closely-packed molecules with low kinetic energy, resulting in structural rigidity and resistance to external forces. Unlike liquids or gases, solids maintain their shape and do not fill the entire available volume. The atoms in a solid are tightly bound in a regular or irregular lattice formation. Solids cannot be compressed easily, in contrast to gases, which have loosely-packed molecules.

Amorphous solid: An amorphous solid lacks the long-range order seen in crystals. It is sometimes referred to as glass or glassy solid, especially when it undergoes a glass transition. Examples include glasses, metallic glasses, plastics, and polymers.

Liquid: A liquid is a fluid that takes the shape of its container and maintains a constant volume. It is one of the four fundamental states of matter, possessing definite volume but no fixed shape.

Gas: Gas is a fundamental state of matter, alongside solids, liquids, and plasmas.

Vapor: Vapor refers to a substance in gas phase below its critical temperature. It can condense to a liquid with increased pressure, without reducing its temperature. Unlike aerosols, vapor does not contain suspended particles of liquid or solid within a gas.

Plasma (physics): Plasma is a fundamental state of matter abundant in the universe, consisting of charged particles like ions and electrons. It is prevalent in stars, as well as in the rarefied intracluster and intergalactic mediums. It can be produced artificially through heating or subjecting a neutral gas to an electromagnetic field.

Degenerate matter: Degenerate matter, occurring at low temperatures, arises when the Pauli exclusion principle profoundly affects the state of matter. It is observed in dense celestial objects like white dwarfs and neutron stars, where gravitational collapse cannot be averted through thermal pressure alone. Furthermore, this concept is applicable to metals in the Fermi gas approximation.

Bose–Einstein condensate: A Bose–Einstein condensate (BEC) is a state of matter formed when a gas of bosons is cooled to very low temperatures. At this state, a significant number of bosons occupy the lowest quantum state, resulting in macroscopic quantum effects like wavefunction interference. This condensation phenomenon is also observed in superconductors, where Cooper pairs form a condensate, indicating a phase transition.

Phase transition: A phase transition is the process of transitioning between different states of matter, such as solid, liquid, gas, and sometimes plasma. It occurs when external conditions like temperature or pressure change, causing certain properties of the substance to also change. These transitions can be abrupt and are defined by specific conditions.

Phase (matter): In physical sciences, a phase refers to a chemically uniform and physically distinct region of material that can be mechanically separated. Each phase in a system has its own unique properties. For example, in a glass jar containing ice, water, and humid air, the ice cubes, water, and air are distinct phases. The glass jar itself is also considered a separate phase.

Phase diagram: A phase diagram is a chart showing conditions where different phases exist together in equilibrium. It is used in physical chemistry, engineering, mineralogy, and materials science to understand thermodynamic distinctions and coexistence of phases.

Boiling: Boiling is the rapid transition of a liquid into gas or vapor, caused by heating it to its boiling point. It happens when the vapor pressure of the liquid equals the surrounding atmospheric pressure. This process is the opposite of condensation and is one of the main ways that liquids evaporate.

Melting: Melting, or fusion, is a process where a solid substance transitions to a liquid state due to increased internal energy from heat or pressure. This occurs at the substance's melting point, disrupting the solid's structured arrangement and resulting in a less ordered liquid state.

Freezing: Freezing is the process in which a liquid transforms into a solid when cooled below its freezing point. It refers to the solidification phase change of a liquid or its content due to cooling.

Evaporation: Evaporation is a process where a liquid changes into a gas on its surface. It is affected by surrounding gas concentration and humidity. Energy transfer between liquid molecules causes them to escape into the air as a gas. Evaporation also cools the liquid by removing its energy.

Condensation: Condensation is the transformation from gas to liquid, which is the opposite of vaporization. It primarily pertains to the water cycle and occurs when water vapor comes into contact with a liquid, solid surface, or cloud condensation nuclei. When the conversion directly happens from gas to solid, it is known as deposition.

Ionization: Ionization is the process of atoms or molecules gaining or losing electrons to acquire a positive or negative charge. It can occur through various means, such as collisions with particles or electromagnetic radiation. This results in the formation of ions, which can also be created through reactions or radioactive decay.

Sublimation (phase transition): Sublimation is a phase transition where a substance goes from solid to gas without becoming a liquid. The reverse process is called deposition. The verb form is sublime, and the product obtained is called sublimate.

Critical point (thermodynamics): A critical point in thermodynamics is the endpoint of a phase equilibrium curve, such as the liquid-vapor critical point. This point signifies the conditions under which a liquid and its vapor can coexist. At the critical point, phase boundaries disappear due to a specific critical temperature and pressure. Other examples of critical points include those in mixtures and transitions between ferromagnetism and paramagnetism without an external magnetic field.

Triple point: The triple point is the temperature and pressure at which a substance's three phases coexist in equilibrium. It represents the meeting point of sublimation, fusion, and vaporization curves. For mercury, the triple point is at -38.8°C and a pressure of 0.165 mPa.

Electricity: Electricity is the physical phenomenon linked to the presence and movement of charged matter. It is closely connected to magnetism and falls under the umbrella of electromagnetism. Notable examples of electricity include lightning, static electricity, and electric heating.

Magnetism: Summary: Magnetism refers to the physical properties related to a magnetic field, enabling objects to attract or repel each other. It is a fundamental aspect of electromagnetism, arising from the magnetic fields generated by both electric currents and magnetic moments of elementary particles.

Magnet: A magnet is a material that creates an invisible magnetic field, which exerts a force on other ferromagnetic materials, attracting or repelling them, including iron, steel, nickel, and cobalt.

Electromagnet: An electromagnet is a magnet created by an electric current. It is made of wire wound into a coil, and the magnetic field is concentrated in the center of the coil. When the current is turned off, the magnetic field disappears. Electromagnets can be enhanced by using a magnetic core made of iron or other materials to increase their strength.

Dipole: A dipole is an electromagnetic phenomenon that involves the separation of positive and negative charges. It can occur in two ways: electric dipole deals with the separation of charges, while magnetic dipole involves the circulation of an electric current. Examples include a pair of charges and a loop of wire with constant current. A bar magnet is an example of a magnet with a permanent magnetic dipole moment.

Metamaterial: Metamaterials are engineered materials that possess unique properties not found in natural materials. They consist of composite elements made from metals and plastics, arranged in repeating patterns at a small scale. Unlike traditional materials, metamaterials derive their properties from their specific structures, allowing them to manipulate electromagnetic waves. By blocking, absorbing, enhancing, or bending waves, they offer advantages beyond what conventional materials can achieve.

Oil drop experiment: The oil drop experiment, conducted in 1909 by Robert A. Millikan and Harvey Fletcher, determined the elementary electric charge. This groundbreaking experiment was carried out at the Ryerson Physical Laboratory, University of Chicago, and earned Millikan the Nobel Prize in Physics in 1923.

Photoelectric effect: The photoelectric effect is the release of electrons when light hits a material, forming photoelectrons. It is studied in condensed matter physics, quantum chemistry, and solid state to understand the properties of atoms, molecules, and solids. This effect is employed in electronic devices for light detection and controlled electron emission.

Electrostatics: Electrostatics is the study of stationary or slow-moving electric charges.

Coulomb's law: Coulomb's law is an experimental physics law that calculates the force between two stationary charged particles. It was published by physicist Charles-Augustin de Coulomb in 1785 and is fundamental to the theory of electromagnetism. This law, also known as the inverse-square law, allows for discussions on the amount of electric charge in a particle.

Electric charge: Electric charge is a fundamental property of matter that generates forces in the presence of electromagnetic fields. It comes in two types: positive and negative. Like charges repel, unlike charges attract. Objects with no net charge are called electrically neutral. This knowledge is known as classical electrodynamics, accurately explaining interactions between charged substances.

Electric field: An electric field is a physical field created by charged particles that exerts attractive or repulsive forces on other charged objects. These forces are described by Coulomb's Law, with greater charges resulting in stronger forces. Electric fields originate from charges and time-varying electric currents, and are part of the electromagnetic field, one of the four fundamental forces of nature.

Electric potential: Electric potential is the work energy required to move a small charge from a reference point to a specific point in an electrical field. It represents the energy per unit charge for a negligible disturbance in the field. The motion should proceed without acceleration or radiation, and the electric potential at the reference point is zero. Earth or any point can serve as the reference point.

Dielectric: A dielectric is an insulating material that can be polarized by an electric field. Unlike conductors, it does not allow the flow of charges but instead causes a slight shift in the positions of charges, resulting in polarization. This polarization creates an internal electric field that reduces the overall field within the dielectric.

Magnetostatics: Magnetostatics is the study of steady magnetic fields in systems. It is equivalent to electrostatics, but for stationary charges. The equations of magnetostatics can even predict fast magnetic switching events. It is widely used in micromagnetics applications, such as modeling magnetic storage devices in computer memory.

Magnetic field: A magnetic field is a vector field that influences moving electric charges, electric currents, and magnetic materials. It causes a force on a moving charge and attracts or repels magnets and ferromagnetic materials. It also exerts small forces on nonmagnetic materials through paramagnetism, diamagnetism, and antiferromagnetism. Magnetic fields are present around magnetized materials, electric currents, and varying electric fields. They are mathematically described as a vector field assigning a vector to each point in space.

Ferromagnetism: Ferromagnetism is a property of certain materials that allows them to form permanent magnets due to their significant magnetic permeability and coercivity. These materials are noticeably attracted to magnets and can induce temporary magnetization when exposed to an external magnetic field. The acquisition of permanent magnetization by a material depends on the strength of the applied field and the coercivity of the ferromagnetic material.

Maxwell's equations: Maxwell's equations, or Maxwell-Heaviside equations, are a set of mathematical equations that describe the behavior of electric and magnetic fields. They form the basis of classical electromagnetism, optics, and electric circuits. These equations provide a model for various technologies like power generation, wireless communication, radar, and more. They explain how charges, currents, and changes in fields generate electric and magnetic fields. They were named after James Clerk Maxwell, who first proposed that light is an electromagnetic phenomenon and Oliver Heaviside, who contributed to their modern formulation.

Lorentz force: The Lorentz force is the resultant force experienced by a charged particle moving in an electric and magnetic field. It encompasses the combined effect of electromagnetic forces on the particle's motion.

Electromagnetic field: An electromagnetic field represents the effects of electric charges. It consists of an electric field and a magnetic field. Maxwell's equations explain how charges and currents interact with the field, while the Lorentz force law describes the forces experienced by charges in electric and magnetic fields. Changes in the fields influence each other. Disturbances in the electric field create disturbances in the magnetic field, resulting in the propagation of electromagnetic waves through space.

Electric current: An electric current is the flow of charged particles, like electrons or ions, through an electrical conductor. It represents the net rate of electric charge movement across a surface. The type of charge carriers depends on the conductor, such as electrons in wires, electrons or holes in semiconductors, ions in electrolytes, and ions and electrons in plasma.

Alternating current: Alternating current (AC) is an electric current that changes direction and magnitude continuously over time. It is the primary form of electrical energy used in businesses and homes, powering appliances and devices when plugged into a wall socket. AC is distinguished from direct current (DC) which flows in a single direction, and the abbreviations AC and DC are commonly used to refer to these currents.

Direct current: Direct current (DC) refers to the one-directional flow of electric charge. It can flow through various materials like conductors, semiconductors, and insulators, or even through a vacuum. DC is distinct from alternating current (AC), as it flows in a constant direction, and was previously known as galvanic current. Examples of DC power sources include electrochemical cells.

Electromagnetic induction: Electromagnetic induction is the creation of emf in a conductor due to a changing magnetic field.

Electromotive force: Electromotive force (emf) is the energy transferred to an electric circuit per unit of charge, measured in volts. It is provided by electrical transducers and devices like batteries and generators that convert different forms of energy into electrical energy. Emf is not a physical force, and the term has been deprecated in favor of source voltage or source tension.

Eddy current: An eddy current is an electric current induced in a conductor by a changing magnetic field or the movement of a conductor in a magnetic field. It flows in closed loops perpendicular to the magnetic field. Eddy currents can be generated in nearby conductors by AC electromagnets or transformers. The current's magnitude depends on the magnetic field strength, loop area, flux change rate, and material resistivity. They create circular patterns in metal resembling eddies or whirlpools in a liquid.

Electromagnetic radiation: Electromagnetic radiation (EMR) is the propagation of waves in the electromagnetic field, carrying both momentum and electromagnetic radiant energy. It encompasses various types such as radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, and gamma rays, forming the electromagnetic spectrum.

Electromagnetic spectrum: The electromagnetic spectrum refers to the entire range of different types of electromagnetic radiation. It is divided into several bands, each named after the waves within them based on their frequency. These include radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, and gamma rays. The waves in each band have unique characteristics, production methods, interactions with matter, and practical applications.

Radio wave: Radio waves are a type of electromagnetic radiation with long wavelengths, ranging from 1mm at 300 GHz to over 10,000 kilometers at 30 Hz. They travel at the speed of light and are generated by charged particles undergoing acceleration. Lightning, astronomical objects, and warm objects emit natural radio waves.

Microwave: Microwaves are a type of electromagnetic radiation with wavelengths ranging from 30 cm to 1 mm. They have frequencies between 1 GHz and 300 GHz and are commonly defined as the range between 1 and 100 GHz. Microwaves include the entire SHF band and are often referred to by radar band designations such as S, C, X, Ku, K, or Ka band.

Infrared: Infrared refers to electromagnetic radiation between microwaves and visible light that is invisible to the human eye. It covers wavelengths from approximately 750 nm to 1000 μm.

Visible spectrum: The visible spectrum refers to the range of electromagnetic radiation that can be seen by the human eye, commonly known as visible light. It may also include ultraviolet and infrared radiation, depending on the definition used.

Light: Light is electromagnetic radiation detected by the human eye. It falls within the 400–700 nm wavelength range, equivalent to frequencies of 750–420 THz. It encompasses the visible spectrum between infrared and ultraviolet.

Ultraviolet: Ultraviolet (UV) is a type of electromagnetic radiation that falls between visible light and X-rays. It makes up about 10% of the Sun's radiation and can also be produced by electric arcs, Cherenkov radiation, and certain light sources like mercury-vapor lamps and tanning lamps.

X-ray: X-ray is a powerful form of electromagnetic radiation, also known as Röntgen radiation, named after its discoverer Wilhelm Conrad Röntgen. It was identified in 1895 and is called X-radiation due to its mysterious nature.

Gamma ray: Gamma rays are a type of electromagnetic radiation that comes from the decay of atomic nuclei. They have the shortest wavelength and highest energy among electromagnetic waves. French chemist Paul Villard discovered gamma radiation in 1900, and they were named by Ernest Rutherford due to their strong ability to penetrate matter.

Bremsstrahlung: Bremsstrahlung is electromagnetic radiation emitted by a charged particle when it slows down due to interaction with another charged particle. This energy loss is converted into radiation, following the conservation of energy. The process produces a continuous spectrum of radiation, with higher frequencies increasing as the deceleration energy rises.

Fluorescence: Fluorescence is the emission of light by a substance that has absorbed light or electromagnetic radiation. It is a type of luminescence where the emitted light has a longer wavelength and lower energy than the absorbed radiation. This phenomenon is observable when a substance exposed to ultraviolet radiation emits visible light, giving it a distinct color. Unlike phosphorescence, fluorescence ceases quickly after the radiation source is removed.

Electrical network: An electrical network is a connection of electrical components, forming a model of interconnection. It can be represented by electrical elements. Not all networks are circuits, although all circuits are networks. Linear electrical networks only consist of sources, lumped elements, and distributed elements. These networks allow for linear superimposition of signals and can be analyzed using Laplace transforms to determine DC, AC, and transient responses.

Electrical impedance: Electrical impedance refers to the total resistance and reactance that opposes the flow of alternating current in a circuit. It is a vital concept in electrical engineering.

Electrical resistance and conductance: Electrical resistance is the measure of opposition to electric current, while conductance measures the ease of current flow. Resistance is similar to mechanical friction. The units for resistance and conductance are ohm and siemens, respectively.

Ohm's law: Ohm's law is a fundamental principle in physics that relates the electric current flowing through a conductor to the voltage across it. It states that the current is directly proportional to the voltage, with resistance as the constant of proportionality. This law is described by three mathematical equations.

Semiconductor: A semiconductor is a material with electrical conductivity between a conductor and an insulator. Its resistivity decreases as temperature rises. Impurities can be introduced to alter its conducting properties. Semiconductors form junctions, which are the basis for diodes, transistors, and modern electronics. Common examples include silicon and germanium. Gallium arsenide is the second-most common semiconductor, used in lasers, solar cells, and integrated circuits. Silicon plays a crucial role in electronic circuit fabrication.

Superconductivity: Superconductivity is a phenomenon in specific materials where electrical resistance disappears and magnetic fields are repelled. These materials, known as superconductors, have a critical temperature below which their resistance abruptly drops to zero. They can maintain an electric current indefinitely without the need for a power source.

Capacitance: Capacitance refers to an object or device's ability to store electric charge, measured by its response to a difference in electric potential. It has two main forms: self capacitance, which measures potential between an object and ground, and mutual capacitance, which measures potential between two components. Mutual capacitance is crucial in capacitors, electronic components that add capacitance to circuits.

Gain (electronics): Gain in electronics refers to the ability of a two-port circuit to amplify a signal by adding energy from a power supply. It is measured as the ratio of signal power at the output to the input, often expressed in dB. Active components or circuits have a gain greater than one, while passive circuits have a gain less than one.

Inductance: Inductance is the property of an electrical conductor that resists changes in the electric current flowing through it. It is caused by the production of a magnetic field around the conductor, which depends on the current's magnitude. According to Faraday's law of induction, any change in magnetic field induces an electromotive force (EMF) in the circuit, known as electromagnetic induction. This induced voltage, called back EMF, opposes the change in current, as stated by Lenz's law.

Kirchhoff's circuit laws: Kirchhoff's circuit laws, established by Gustav Kirchhoff in 1845, are fundamental principles in electrical engineering that relate to current and potential difference in electrical circuits. They expanded upon the work of Georg Ohm and preceded James Clerk Maxwell's contributions. Known as Kirchhoff's rules or laws, they are widely used in network analysis and apply to both time and frequency domains.

Mechanics: Mechanics is a branch of math and physics that studies the interactions between force, matter, and motion in physical objects. It explains how forces cause objects to move or change position.

Ballistics: Ballistics is the study of projectiles launched from weapons, such as bullets and rockets. It involves understanding their flight behavior, impact effects, and designing them for optimal performance.

Continuum mechanics: Continuum mechanics is a field of mechanics that studies the deformation and force transmission in continuous materials rather than individual particles. It was initially developed by Augustin-Louis Cauchy in the 19th century.

Classical mechanics: Classical mechanics is a physical theory that explains the movement of large objects like projectiles, machinery parts, and celestial bodies. It differs from modern physics and was a significant shift in the field's methods and philosophy.

Flight: Flight is the movement of an object without touching any surface, occurring in the atmosphere or in outer space. It can be achieved through aerodynamic lift, buoyancy, or ballistic movement.

Kinematics: Kinematics is a subfield of physics that describes motion without considering the forces involved. It is often referred to as the "geometry of motion" and can be seen as a branch of mathematics. Kinematics uses geometry to determine the position, velocity, and acceleration of objects based on initial conditions. Forces are not considered in kinematics, as they fall within the realm of kinetics. For more information, refer to the field of analytical dynamics.

Relativistic mechanics: Relativistic mechanics encompasses mechanics compatible with special relativity (SR) and general relativity (GR), allowing for a description of particles or fluids moving at speeds comparable to the speed of light. It extends classical mechanics to high velocities and energies and includes electromagnetism, which was not possible in Galilean relativity. Special and general relativity serve as the foundations for relativistic mechanics. The unification of SR with quantum mechanics is known as relativistic quantum mechanics, while quantum gravity attempts to unify GR with quantum mechanics and remains an unsolved problem in physics.

Statics: Statics is a branch of mechanics that studies the forces and torque acting on a system in equilibrium with its surroundings, without any acceleration.

Oscillation: Oscillation is the repetitive variation of a measure around a central value or between different states. It is observed in phenomena like swinging pendulums and alternating current. In physics, oscillations are utilized to approximate complex interactions, such as atomic behavior.

Harmonic oscillator: A harmonic oscillator is a classical mechanics system that experiences a restoring force when displaced from its equilibrium position. The force is proportional to the displacement.

Simple harmonic motion: Simple harmonic motion refers to a periodic motion of an object caused by a restoring force. This force is proportionate to the object's distance from its equilibrium position and always acts towards it. The resulting oscillation follows a sinusoidal pattern, continuing indefinitely.

Quantum mechanics: Quantum mechanics is a physics theory that explains natural behavior at atomic and smaller scales. It forms the basis of various quantum fields like chemistry, technology, and information science.

Pauli exclusion principle: The Pauli exclusion principle, formulated by Austrian physicist Wolfgang Pauli, states that in quantum mechanics, identical particles with half-integer spins cannot occupy the same quantum state. This principle applies to electrons and other fermions and is a fundamental concept in quantum mechanics.

Quantum chromodynamics: Quantum chromodynamics (QCD) is the theory explaining the strong interaction between quarks using gluons. It is a non-abelian gauge theory with symmetry group SU(3) and plays a crucial role in the Standard Model. QCD introduces the concept of color as the analog of electric charge, and gluons act as force carriers. A significant amount of experimental evidence supports QCD.

Quantum electrodynamics: Quantum electrodynamics (QED) is a particle physics theory that combines quantum mechanics and special relativity to explain the interaction between light and matter. It mathematically describes the exchange of photons between charged particles, providing a complete understanding of how matter and light interact.

Quantum entanglement: Quantum entanglement is a phenomenon where particles become interconnected, sharing a quantum state that cannot be described independently. This property, absent in classical mechanics, is crucial to understanding quantum physics.

Quantum field theory: Quantum field theory (QFT) is a theoretical framework in physics that combines classical field theory, special relativity, and quantum mechanics. It is used to construct models of subatomic particles in particle physics and models of quasiparticles in condensed matter physics.

Quantum gravity: Quantum gravity is a field of physics that combines gravity with quantum mechanics. It explores how gravity behaves on a small scale, like near black holes, neutron stars, and during the early universe after the Big Bang.

Quantum tunnelling: Quantum tunnelling is a phenomenon in physics where particles, like electrons or atoms, can traverse through energy barriers that would typically be impassable based on classical mechanics. This occurs despite the particles lacking the necessary energy to overcome the obstacle.

Schrödinger equation: The Schrödinger equation is a fundamental equation in quantum mechanics that describes the behavior of particles. It was developed by Erwin Schrödinger in 1925 and is named after him. This equation, a linear partial differential equation, governs the wave function of quantum systems. Its discovery was a significant milestone in the advancement of quantum mechanics and led to Schrödinger receiving the Nobel Prize in Physics in 1933.

Uncertainty principle: The Uncertainty Principle in quantum mechanics, also known as Heisenberg's indeterminacy principle, sets a limit on the precision of simultaneously knowing pairs of physical properties like position and momentum. In simple terms, the more accurately we measure one property, the less accurately we can know the other.

Wave function: A wave function is a mathematical description of the quantum state of a system. It uses complex numbers to assign probabilities to different positions in space. The Born rule allows us to interpret these probabilities as actual measurements. The wave function must be normalized, meaning that its integral over all positions is equal to 1. However, only relative phase and magnitude can be measured, not absolute values or directions. To obtain measurable quantities, we apply quantum operators to the wave function and calculate statistical distributions.

Wave–particle duality: Wave-particle duality is the idea in quantum mechanics that quantum entities can behave as both particles and waves depending on the experimental conditions. Classical concepts like particle and wave fail to fully explain their behavior. This concept emerged from the discovery that light acts as both a wave and a particle, and electrons exhibit both particle-like and wave-like characteristics. Duality was introduced to describe these contradictions.

Hamiltonian mechanics: Hamiltonian mechanics, introduced by Sir William Rowan Hamilton, is a reformulation of Lagrangian mechanics from 1833. It replaces velocities with momenta and describes classical mechanics while interpreting the same physical phenomena.

Lagrangian mechanics: Lagrangian mechanics is a branch of classical mechanics that is based on the principle of stationary action. It was developed by Joseph-Louis Lagrange in the late 18th century.

Mass: Mass is an intrinsic property of a body, not solely determined by the quantity of matter. Different atoms and particles can have the same amount of matter but different masses. In modern physics, mass has multiple definitions but is experimentally measured as a body's inertia and determines its gravitational attraction to other bodies.

Density: Density is a substance's mass per unit volume, often represented by ρ or D. It is calculated as mass divided by volume.

Conservation of mass: The law of conservation of mass states that in a closed system, mass cannot be created or destroyed. It remains constant over time, as neither matter nor energy can be added or removed from the system. Hence, the quantity of mass is conserved.

Momentum: Momentum is a vector quantity in Newtonian mechanics calculated as the product of an object's mass and velocity. It represents the object's magnitude and direction. The formula for momentum is represented as p = mv.

Force: Force is a physical influence that can alter an object's velocity, causing it to accelerate. It is mathematically defined and includes the notions of pushing, pulling, and counteractions. Force is a vector quantity, with magnitude and direction being significant. The unit of force is the newton (N) and it is denoted by the symbol F.

Motion: Motion is the change in position of an object over time. It can be described using various measurements such as displacement, distance, velocity, acceleration, and speed. The field of kinematics studies motion without considering its cause, while dynamics explores the impact of forces on motion.

Newton's laws of motion: Newton's laws of motion are three fundamental laws that define the relationship between an object's motion and the forces acting upon it. These laws explain that an object will remain at rest or move in a straight line at a constant speed unless acted upon by a force. The net force on an object is equal to its mass multiplied by its acceleration, or the rate of change in momentum with time. Additionally, if two bodies interact, the forces they exert on each other have equal magnitudes but opposite directions.

Speed: Speed refers to the measure of how fast an object is moving, and is represented by the magnitude of its position change over time. It is a scalar quantity indicating only the magnitude of motion, unlike velocity which also determines direction. Average speed is calculated as distance divided by time, while instantaneous speed is the limit of average speed as time interval approaches zero.

Velocity: Velocity is the measure of an object's speed and direction of motion. It is a fundamental concept in kinematics, a branch of classical mechanics that explains body motion.

Acceleration: Acceleration refers to the change in velocity of an object over time. It is a component of kinematics, which studies motion. Acceleration is a vector quantity and its direction is determined by the net force acting on the object. Its magnitude depends on the net resulting force and the object's mass, as described by Newton's Second Law.

Equations of motion: Equations of motion describe the behavior of a physical system's motion over time. They use mathematical functions to define the system's dynamics, often involving spatial coordinates, time, and momentum. These equations can be solved to understand how the system moves based on known dynamics, either in classical mechanics using Euclidean space or in relativity using curved spaces.

Kinetic energy: Kinetic energy is the energy an object has because it is moving.

Potential energy: Potential energy is the stored energy possessed by an object due to its position, internal stresses, electric charge, or other factors. The concept was named by William Rankine but can be traced back to Aristotle's idea of potentiality.

Inertia: Inertia is a fundamental principle in physics, described by Isaac Newton as objects in motion staying in motion and objects at rest staying at rest unless acted upon by a force. It is a manifestation of mass and is governed by Newton's first law of motion. The summary is concise, focusing on the key features of inertia.

Moment of inertia: The moment of inertia is a property of a rigid body that determines the torque needed for a desired change in rotation. It depends on the body's mass distribution and the chosen axis, with larger moments requiring more torque to alter the rate of rotation.

Friction: Friction is a force that opposes the movement of surfaces, layers, and elements sliding against each other. It has various types, including dry, fluid, lubricated, skin, and internal friction.

Impulse (physics): Impulse in physics refers to the change in momentum of an object. It is denoted as J and occurs when the initial momentum of an object, p1, is transformed into a subsequent momentum, p2.

Power (physics): Power in physics refers to the rate at which energy is transferred or converted. It is measured in watts, which is equal to one joule per second. Power is also known as activity in some older works and is considered a scalar quantity.

Work (physics): Work in physics refers to the transfer of energy to or from an object through the application of force along a displacement. It is calculated by multiplying the force strength by the distance traveled. If the force aligns with the direction of motion, it does positive work, while if it opposes the displacement, it does negative work.

Angular momentum: Angular momentum is the rotational counterpart of linear momentum. It is a conserved quantity, meaning it remains constant in a closed system. This property gives rise to the useful characteristics of objects like bicycles, flying discs, and gyroscopes. Angular momentum conservation explains the formation of spirals in hurricanes and the high rotational rates of neutron stars. While conservation determines the possible motion of a system, it does not uniquely define it.

Centripetal force: Centripetal force is a force that directs a body along a curved path. It always points towards the center of the path and is described by Isaac Newton as a force that draws bodies towards a center point. Gravity acts as the centripetal force in Newtonian mechanics, enabling astronomical orbits.

Rotation: Rotation refers to the circular movement of an object around an axis. It can occur in either a clockwise or counterclockwise direction. A plane figure rotates around a perpendicular axis intersecting at a center of rotation, while a solid figure can rotate around an infinite number of axes and angles, including chaotic rotation.

Torque: Torque, or moment of force, is the rotational equivalent of linear force in physics and mechanics. It measures the rate of change of angular momentum transmitted to a single object.

Weight: Weight is the force acting on an object in response to acceleration or gravity in science and engineering.

Weighing scale: A weighing scale is a device used to measure weight or mass. It is also known as a scale, balance, mass scale, or weight balance.

Frame of reference: A frame of reference is an abstract coordinate system used in physics and astronomy. It includes an origin, orientation, and scale specified by reference points, which are geometric points identified mathematically and physically.

Newton's law of universal gravitation: Newton's law of universal gravitation states that all particles in the universe attract each other with a force that depends on their masses and the distance between them. This law unifies the understanding of gravity on Earth with astronomical behaviors.

Coriolis force: The Coriolis force is an inertial force that affects objects in motion in a rotating frame of reference. It deflects objects to the left or right depending on the direction of rotation. The deflection caused by this force is known as the Coriolis effect. The mathematical expression for the Coriolis force was introduced by French scientist Gaspard-Gustave de Coriolis in 1835, in relation to water wheel theory. The term Coriolis force became commonly used in meteorology during the 20th century.

Fluid mechanics: Fluid mechanics is a physics branch that studies fluids and the forces acting on them. It applies to various fields like engineering, geophysics, biology, and meteorology.

Bernoulli's principle: Bernoulli's principle is a fundamental concept in fluid dynamics that explains the relationship between pressure, speed, and height in a fluid. It states that as fluid speed increases, static pressure decreases. The principle was first introduced by Daniel Bernoulli in 1738 and later derived as Bernoulli's equation by Leonhard Euler in 1752.

Buoyancy: Buoyancy, also known as upthrust, is the upward force exerted by a fluid that opposes the weight of an immersed object. The pressure at the bottom of a fluid column or submerged object is higher than at the top due to the weight of the overlying fluid. This pressure difference creates a net upward force on the object, equivalent to the weight of the fluid that would occupy the submerged volume.

Convection: Convection is the spontaneous flow of fluid caused by differences in material properties and body forces, mainly density and gravity. It can occur in both single or multiphase fluids and is often influenced by thermal expansion and buoyancy. Convection can also happen in soft solids or mixtures where particles can move.

Drag (physics): Drag is a force that opposes the motion of an object in relation to a fluid it is moving through. This force can occur between fluid layers or between a fluid and a solid surface.

Diffusion: Diffusion is the movement of substances from areas of high concentration to areas of low concentration, driven by differences in energy levels. It can occur in both directions, including from low to high concentration. This random process is widely used to model various real-life scenarios and is applied in fields like physics, finance, and marketing.

Fluid dynamics: Fluid dynamics is a field in physics, physical chemistry, and engineering that studies the movement of liquids and gases. It includes subdisciplines like aerodynamics and hydrodynamics. This branch of fluid mechanics has various applications, such as calculating forces on aircraft, predicting weather patterns, and understanding phenomena in space.

Aerodynamics: Aerodynamics is the study of air motion, especially around solid objects like airplane wings. It is a branch of fluid dynamics that is important in aeronautics. While gas dynamics is similar, aerodynamics specifically refers to air. The formal study began in the 18th century, with efforts mainly focused on achieving flight. Over time, the use of mathematical analysis, wind tunnels, and simulations has advanced flight and other technologies. Current research emphasizes compressible flow, turbulence, and computational methods.

Lift (force): Lift is the force exerted by a fluid on an object as it flows around it. It is perpendicular to the flow direction and counters gravity. It can act in any direction.

Navier–Stokes equations: The Navier–Stokes equations are partial differential equations that explain the behavior of viscous fluids. They were named after Claude-Louis Navier and George Gabriel Stokes, who came up with them between 1822 and 1850.

Osmosis: Osmosis is the movement of solvent molecules through a membrane from high to low water potential, equalizing solute concentrations. It can also describe solvent movement between solutions of different concentrations. Osmosis can be used to do work, and osmotic pressure is the external pressure required to stop solvent movement. Osmotic pressure depends on solute concentration, not identity.

Reynolds number: The Reynolds number is a dimensionless quantity used in fluid mechanics to predict fluid flow patterns. It compares inertial and viscous forces to determine if the flow is laminar or turbulent. Laminar flow occurs at low Reynolds numbers, while turbulent flow occurs at high Reynolds numbers. Turbulence is caused by differences in speed and direction of the fluid, which leads to churning and energy loss. The chances of cavitation, or the formation of vapor-filled cavities, increase with higher Reynolds numbers in liquids.

Surface tension: Surface tension is the property of a liquid surface to minimize its area, resulting in a shrinkage. This phenomenon enables objects denser than water, like razor blades and insects, to float without submerging.

Turbulence: Turbulence is a chaotic fluid motion that involves unpredictable changes in pressure and flow velocity. Unlike laminar flow where fluid flows smoothly in parallel layers, turbulence causes disruptions between these layers.

Viscosity: Viscosity measures a fluid's resistance to deformation at a specific rate. It corresponds to the thickness of liquids, with syrup having higher viscosity than water. Scientifically, viscosity is defined as force multiplied by time divided by area, and its SI units are newton-seconds per square meter, or pascal-seconds.

Solid mechanics: Solid mechanics is a field within continuum mechanics that examines how solid materials respond to various forces, temperature fluctuations, phase alterations, and other internal and external factors, by studying their motion and deformation.

Deformation (engineering): Deformation in engineering refers to changes in size or shape of an object. Displacement measures absolute change in position, while deflection is the relative change in external displacements. Strain expresses internal changes in shape and is related to stress by a curve. The relationship between stress and strain is linear until the yield point, beyond which permanent distortion occurs, known as plastic deformation. The study of stress and strain is important in strength of materials and structural analysis.

Elastic modulus: The elastic modulus measures an object or substance's ability to resist elastic deformation when stressed.

Elasticity (physics): Elasticity is the property of a substance to resist deformation and recover its original shape when the applied force is removed. Unlike plasticity, where the substance remains permanently deformed, elastic materials return to their initial size and shape after being distorted.

Fatigue (material): Fatigue in materials refers to the formation and spreading of cracks caused by repetitive loading. As the cracks grow, striations appear on the fracture surface. Eventually, the crack reaches a critical size, leading to rapid propagation and complete fracture when the stress intensity exceeds the material's fracture toughness.

Hooke's law: Hooke's law is an empirical law in physics that states the force needed to extend or compress a spring is proportional to the displacement of the spring from its equilibrium position. The law is named after Robert Hooke, a 17th-century British physicist who first stated it in 1676. The relationship is expressed as Fs = kx, where Fs is the force, k is a constant factor characterizing the spring, and x is the displacement. Hooke's law has been known since 1660 and provides a fundamental understanding of how springs behave.

Plasticity (physics): Plasticity in physics and materials science refers to a solid material's capability to undergo permanent deformation when subjected to external forces. This non-reversible change of shape, such as bending or pounding metal, occurs due to internal modifications within the material. In engineering, the shift from elastic behavior to plastic behavior is called yielding.

Stiffness: Stiffness: The measure of how an object opposes deformation when subjected to a force.

Strength of materials: Strength of materials is a field that analyzes stresses and strains in structural components like beams, columns, and shafts. It uses properties such as yield strength, ultimate strength, and Young's modulus to predict structural response and failure modes. Important factors considered include material properties, geometry changes, and boundary constraints.

Stress (mechanics): Stress (mechanics) is a term in continuum mechanics that represents the forces present during deformation. It relates to the stretching or shortening of an object due to pulling apart or pushing together. Stress depends on the magnitude of force and the area on which it acts. It is quantified as force per unit area, measured in newtons per square meter (N/m2) or pascal (Pa).

Statistical mechanics: Statistical mechanics is a mathematical framework in physics that uses statistical methods and probability theory to understand the behavior of large groups of tiny particles. It explains how the macroscopic behavior of nature can be derived from the behavior of these ensembles, without assuming or postulating any specific natural laws.

Nuclear physics: Nuclear, the study of atomic nuclei and interactions along with other forms of nuclear matter.

Radioactive decay: Radioactive decay is the emission of radiation energy from an unstable atomic nucleus. Materials with unstable nuclei are considered radioactive. It involves three common types of decay: alpha, beta, and gamma, with beta decay governed by the weak force, and the others by electromagnetism and nuclear force.

Alpha particle: Alpha particles, also known as alpha rays or alpha radiation, are made up of two protons and two neutrons, forming a helium-4 nucleus. They are usually produced during alpha decay but can also be generated in other ways. Named after the Greek letter α, their symbol is α or α2+. Being equivalent to helium nuclei, they can be denoted as He2+ or 42He2+, indicating a helium ion with a +2 charge. Once electrons are acquired, an alpha particle transforms into a regular helium atom 42He.

Beta particle: A beta particle is a high-energy electron or positron released during the radioactive decay of an atomic nucleus through beta decay. Beta decay produces electrons (β−) and positrons (β+).

Nuclear fission: Nuclear fission is a reaction where an atom's nucleus splits into smaller nuclei, emitting gamma photons and releasing a significant amount of energy.

Nuclear fusion: Nuclear fusion is a reaction where atomic nuclei, like deuterium and tritium, combine to form new nuclei, releasing or absorbing energy due to changes in their binding energy. This process powers stars and releases massive amounts of energy.

Nucleosynthesis: Nucleosynthesis is the process of creating atomic nuclei from existing nucleons and nuclei. It began shortly after the Big Bang, resulting in the formation of hydrogen and helium. Later, in stars and their explosions, nucleosynthesis produced the variety of elements and isotopes we have today. The amounts of heavier elements in the universe remain relatively small.

Particle physics: Particle physics (or high-energy physics) is the exploration of the fundamental particles and forces that make up matter and radiation. It investigates the combinations of elementary particles, including protons and neutrons, with a distinct focus on nuclear physics for protons and neutrons.

Standard Model: The Standard Model is a theory in particle physics that explains three of the four fundamental forces and categorizes all known elementary particles. It was developed by scientists over several decades and was further validated by the discovery of quarks, top quarks, tau neutrinos, and the Higgs boson. Additionally, the Standard Model accurately predicts specific properties of weak neutral currents, W and Z bosons.

Physics beyond the Standard Model: Physics beyond the Standard Model (BSM) addresses the shortcomings of the Standard Model by tackling various puzzles. These include unresolved fundamental parameters, the strong CP problem, neutrino oscillations, matter–antimatter asymmetry, and the mysteries surrounding dark matter and dark energy. Moreover, BSM explores the compatibility issues between the Standard Model and general relativity, particularly when facing conditions like the Big Bang or black hole event horizons.

String theory: String theory is a physics framework that replaces point-like particles with one-dimensional strings. These strings move and interact in space, with their properties determined by their vibrations. One of these vibrations gives rise to the graviton, a quantum particle associated with gravity. Therefore, string theory serves as a theory of quantum gravity.

Supersymmetry: Supersymmetry is a theoretical physics framework suggesting a symmetry between particles with different spins. It proposes partner particles for every known particle. Experiments have yet to find evidence for its existence, but if proven, it could explain phenomena like dark matter and the hierarchy problem in particle physics.

Particle: In physical sciences, particles are small objects with physical or chemical properties like volume, density, or mass. They range in size from subatomic particles to microscopic atoms and molecules to macroscopic powders and granular materials. They can also represent larger objects depending on their density, like people in a crowd or celestial bodies in motion.

Scattering: Scattering is a physical phenomenon where particles or radiation are deflected from their original path due to irregularities in the medium they pass through. It includes deviations in reflected radiation. Originally used in reference to light, the concept of scattering expanded to other ray-like phenomena and was linked to heat and acoustic scattering. The discovery of subatomic particles and the application of quantum theory broadened the understanding of scattering and its mathematical frameworks.

Spin (physics): Spin is a type of angular momentum found in elementary particles and composite particles like atoms. Initially believed to be the rotation of a small rigid particle, it is now understood as angular momentum generated by a flow of charge in the electron's wave field. [source: Wikipedia]

Particle accelerator: A particle accelerator is a machine that uses electromagnetic fields to accelerate charged particles to high speeds and energies, creating well-defined beams.

Large Hadron Collider: The Large Hadron Collider (LHC) is a massive particle collider constructed by CERN. It is the largest and most powerful collider in the world, built with the collaboration of thousands of scientists from over 100 countries. The LHC resides in a 27-kilometer tunnel beneath the France-Switzerland border near Geneva, reaching depths of up to 175 meters.

Tevatron: The Tevatron was a particle accelerator in the US, operating from 1983 to 2011. It was the second-highest energy collider ever built, after the LHC. Located in Illinois, it accelerated protons and antiprotons in a 6.28 km ring to energies up to 1 TeV, costing $120 million to complete.

Particle detector: A particle detector, also called a radiation detector, is a device used in physics and engineering to detect, track, and identify ionizing particles. It measures attributes like energy, momentum, charge, and type in addition to detecting particles produced by nuclear decay or cosmic radiation.

Cloud chamber: A cloud chamber is a particle detector that shows ionizing radiation passing through it. It is also called a Wilson cloud chamber.

Subatomic particle: A subatomic particle is a smaller particle than an atom, which can be composite or elementary. It is studied in physics, particularly in particle and nuclear physics. Bosons, like photons or gluons, are force carriers without rest mass or discrete diameter, while fermions cannot overlap or combine and have rest mass.

Hadron: A hadron is a subatomic particle composed of quarks held together by the strong force. They are similar to molecules held together by electric forces. Protons and neutrons, the main components of ordinary matter, are examples of hadrons. The mass of these particles is primarily derived from the binding energy of their constituent quarks, which is attributed to the strong force.

Fermion: Fermions are particles that adhere to Fermi-Dirac statistics. They have half-odd-integer spins (e.g., 1/2, 3/2) and obey the Pauli exclusion principle. This group encompasses quarks, leptons, baryons, and many atoms and nuclei, composed of an odd number of these particles. They are distinctive from bosons, which follow Bose-Einstein statistics.

Lepton: A lepton is an elementary particle with half-integer spin, not affected by strong interactions. There are two types: charged leptons and neutral leptons. Charged leptons can form composite particles like atoms, while neutrinos are rarely observed due to their lack of interaction. The electron is the most well-known lepton.

Electron: An electron is a subatomic particle with a negative electric charge. It is considered elementary because it has no known components or substructure. It is part of the lepton particle family and has a mass of about 1/1836 that of a proton. Electrons have an intrinsic angular momentum and are fermions, meaning two electrons cannot occupy the same quantum state. They display both particle and wave-like behavior, being able to collide with other particles and diffract like light. The wave properties of electrons are easier to observe than those of other particles due to their lower mass and longer de Broglie wavelength.

Muon: A muon is an elementary particle with a charge of -1 e and mass greater than an electron. It is a lepton and considered a fundamental particle, not composed of any simpler particles.

Tau (particle): The tau, also known as the tau lepton or tauon, is an elementary particle similar to the electron. It has a negative electric charge and a spin of 1/2. Like other leptons, it has an antiparticle called the "antitau". Tau particles are denoted by τ−, while the antitau is denoted by τ+.

Neutrino: A neutrino is an electrically neutral fermion that interacts only through weak and gravity forces. It has a very small rest mass, making it nearly massless. Neutrinos do not participate in electromagnetic or strong interactions, allowing them to pass through matter without being detected.

Quark: A quark is an elementary particle that combines to form stable particles called hadrons. Protons and neutrons are the most stable hadrons and make up atomic nuclei. Quarks are never found in isolation due to color confinement, making observations of hadrons crucial in understanding quarks.

Baryon: A baryon is a subatomic particle made up of three quarks, and it belongs to the hadron family. Baryons have odd numbers of valence quarks and exhibit half-integer spin, classifying them as fermions.

Neutron: A neutron is a subatomic particle with no charge and a slightly greater mass than a proton. It, along with protons, makes up the nuclei of atoms. Neutrons and protons behave similarly and are collectively called nucleons. They have a mass of approximately one atomic mass unit. Neutrons are not elementary particles; they consist of three quarks.

Proton: A proton is a stable subatomic particle with a positive charge of +1 e and is symbolized as p or H+. It has a slightly smaller mass than a neutron and is approximately 1,836 times more massive than an electron. Protons, along with neutrons, are called nucleons and are present in atomic nuclei.

Boson: A boson is a subatomic particle with an integer spin quantum number. It is one of the two fundamental classes of particles, with fermions being the other. All observed subatomic particles are either bosons or fermions.

Gauge boson: A gauge boson is a force-carrying elementary particle that enables interactions between elementary particles described by gauge theory. It acts as a mediator exchanging virtual particles.

Photon: A photon is an elementary particle and the fundamental unit of light and other forms of electromagnetic radiation. It is a force carrier for the electromagnetic force and travels at the speed of light in a vacuum. Photons are massless and belong to the boson particle class.

Gluon: A gluon is an elementary particle, massless, with a spin of 1, that mediates the strong interaction between quarks. It binds quarks together, forming particles like protons and neutrons through quantum chromodynamics.

W and Z bosons: The W and Z bosons, also known as weak bosons or intermediate vector bosons, are elementary particles that mediate the weak interaction. The W± bosons have a positive or negative charge and are each other's antiparticles, while the Z0 boson is electrically neutral and is its own antiparticle. These particles have a spin of 1 and are very short-lived. Their discovery was crucial in establishing the Standard Model of particle physics.

Higgs boson: The Higgs boson, or Higgs particle, is a fundamental particle in particle physics theory. It is a massive scalar boson with no electric or color charge, and it couples to mass. The Higgs boson is produced by the quantum excitation of the Higgs field and rapidly decays into other particles.

Meson: A meson is a subatomic particle composed of equal numbers of quarks and antiquarks, bound by the strong interaction. They have a physical size about 0.6 times that of a proton or neutron. Mesons are unstable and decay into lighter mesons and ultimately stable particles like electrons, neutrinos, and photons.

Antimatter: Antimatter is a form of matter made up of antiparticles with reversed charge, parity, and time. It exists naturally in cosmic ray collisions and some radioactive decays. However, only a very small amount has been successfully bound together in experiments to form antiatoms. Generating antimatter is challenging and costly, with total artificial production being minuscule. Despite this, antimatter plays a crucial role in various applications like positron emission tomography, radiation therapy, and industrial imaging.

Antiparticle: In particle physics, every particle has an antiparticle with the same mass but opposite charges. For instance, the electron's antiparticle is the positron, which has a positive charge. Antiparticles are produced naturally in some radioactive decay processes, and their counterparts have reversed charges.

Positron: Positron, the antiparticle of the electron, has a positive charge, equal mass, and a spin of 1/2. Annihilation happens when it collides with an electron, producing multiple photons at low energies.

Theory of relativity: The theory of relativity, developed by Albert Einstein, comprises special relativity (1905) and general relativity (1915). Special relativity applies to all physical phenomena without gravity, while general relativity explains the law of gravitation in relation to the forces of nature. It applies to the cosmological and astrophysical realm, including astronomy.

General relativity: General relativity is Albert Einstein's theory of gravity, which describes gravity as a geometric property of space and time. It refines Newton's law of gravity and is considered the current description of gravitation in modern physics. The theory relates the curvature of spacetime to the energy and momentum of matter and radiation, and is defined by the Einstein field equations.

Special relativity: Special relativity is a scientific theory by Einstein that explains the relationship between space and time. It is based on two postulates: the laws of physics are the same in all frames of reference, and the speed of light is constant for all observers.

Principle of relativity: The principle of relativity states that the laws of physics should appear the same in all frames of reference. This principle requires the equations describing these laws to have the same form regardless of the observer's point of view.

Equivalence principle: The Equivalence principle suggests that gravitational and inertial mass are equivalent. This concept has two forms: the weak form applies to masses of any composition in free fall, while the extended form requires special relativity to hold and for the weak equivalence to be valid everywhere. Albert Einstein's extended form was crucial in developing the theory of general relativity. The strong form requires Einstein's equations to also apply to stellar objects. Extensive experimental tests have been conducted to confirm the principle's accuracy.

Gravitational wave: Gravitational waves are gravity's equivalent of electromagnetic waves, generated by accelerating masses and propagating at the speed of light. Oliver Heaviside proposed them in 1893, and Henri Poincaré confirmed their existence in 1905.

Lorentz transformation: The Lorentz transformations are linear transformations between coordinate frames in spacetime. They relate frames moving at a constant velocity to each other and their inverse is parameterized by the negative of this velocity. These transformations are named after physicist Hendrik Lorentz.

Michelson–Morley experiment: The Michelson-Morley experiment, conducted in 1887, aimed to measure Earth's motion relative to the luminiferous aether, a hypothetical medium thought to carry light waves. Physicists Albert A. Michelson and Edward W. Morley performed the experiment in Cleveland, Ohio, and published their findings later that year.

Causality (physics): Causality in physics refers to the fundamental physical relationship between causes and effects. It is crucial in all natural and behavioral sciences, particularly in physics. Additionally, causality is studied in philosophy, statistics, and logic. In essence, it states that an effect cannot occur without a cause that is within its past light cone, and a cause cannot have an effect beyond its future light cone.

Thermodynamics: Thermodynamics is a physics branch that studies heat, work, and temperature and their relationship to energy, entropy, and matter/radiation properties. It is governed by four laws that describe these quantities using measurable macroscopic properties but can also be understood through statistical mechanics. It finds applications in various scientific and engineering fields, including physical chemistry, biochemistry, chemical engineering, mechanical engineering, and meteorology.

Heat: Heat is the transfer of thermal energy between systems caused by temperature differences. It is often used to refer to thermal energy itself, which is the kinetic energy of atoms in a substance vibrating and colliding.

Temperature: Temperature is a measure of hotness or coldness and is determined using a thermometer. It represents the movement and interaction of atoms in a substance.

Absolute zero: Absolute zero is the lowest possible temperature on the thermodynamic scale, at which the enthalpy and entropy of a cooled ideal gas reach their minimum. This state signifies the absence of any vibrational motion in fundamental particles, except for quantum mechanical, zero-point energy-induced motion. By international agreement, absolute zero is considered as -273.15 degrees Celsius or -459.67 degrees Fahrenheit. The Kelvin and Rankine temperature scales both define their zero points at absolute zero.

Pressure: Pressure is the perpendicular force exerted on an object's surface per area. Gauge pressure is the pressure relative to the ambient pressure.

Laws of thermodynamics: The laws of thermodynamics are scientific laws that define physical quantities and processes in thermodynamic systems at equilibrium. They establish relationships between temperature, energy, entropy, heat, and work. These laws form the basis for understanding phenomena and preclude perpetual motion. They are fundamental laws in physics and are applicable in various natural sciences.

Heat capacity: Heat capacity, or thermal capacity, is a measure of the heat energy needed to raise an object's temperature by one degree. It is measured in joules per kelvin (J/K).

Heat transfer: Heat transfer is a field of thermal engineering that involves the exchange of thermal energy between different physical systems. It includes mechanisms like conduction, convection, radiation, and phase change. Engineers also consider mass transfer for achieving heat transfer. These mechanisms often occur together in the same system.

Entropy: Entropy is a scientific concept related to disorder and uncertainty. It is utilized in various fields, including thermodynamics, statistical physics, information theory, chemistry, physics, biology, cosmology, economics, sociology, weather science, climate change, and information systems.

Internal energy: The internal energy of a thermodynamic system is the total energy contained within it, accounting for energy changes and excluding kinetic and potential energy of the system as a whole. It includes thermal energy and cannot change in an isolated system due to the law of conservation of energy.

Thermodynamic cycle: A thermodynamic cycle is a sequence of processes that transfer heat and work into and out of a system while varying pressure, temperature, and other state variables. It can convert heat into useful work and transfer remaining heat to a cold sink, acting as a heat engine. Conversely, it can use work to move heat from a cold source to a warm sink, acting as a heat pump. If the system is in thermodynamic equilibrium at every point, the cycle is reversible, and the net entropy change is zero.

Thermodynamic free energy: Thermodynamic free energy is a state function in thermodynamics that represents the maximum work a system can perform at constant temperature. Its change indicates if a process is favorable or forbidden. The free energy is not absolute, but relative values and changes are meaningful due to its dependence on a chosen zero point.

Ideal gas law: The ideal gas law is an equation that describes the behavior of a hypothetical ideal gas. It combines Boyle's law, Charles's law, Avogadro's law, and Gay-Lussac's law. Although it has limitations, it is a good approximation for many gases under various conditions.

Enthalpy: Enthalpy is a thermodynamic property that combines a system's internal energy, pressure, and volume. It is commonly used in measurements of chemical, biological, and physical systems at constant pressure. Enthalpy represents the energy, including bond and lattice energies, within a system. It is independent of the path taken to reach its final configuration.

Black-body radiation: Black-body radiation is the emission of thermal electromagnetic radiation by an object in equilibrium. It consists of a continuous spectrum of wavelengths, determined solely by the object's temperature.

Wave: A wave is a dynamic disturbance that propagates in physics, mathematics, engineering, and related fields. It can be periodic, oscillating around an equilibrium value at a certain frequency. Waves can be traveling or standing, depending on their direction of movement. Standing waves have nulls where the amplitude becomes zero. They are commonly described by wave equations for single wave propagation in a defined direction.

Acoustics: Acoustics is the study of mechanical waves in gases, liquids, and solids, including sound, ultrasound, and infrasound. An acoustician is a scientist in this field, while an acoustical engineer specializes in acoustics technology. Acoustics is widely applied in modern society, particularly in audio and noise control industries.

Sound: Sound is a vibration that travels as an acoustic wave through a gas, liquid, or solid. Humans perceive these waves, within the frequency range of 20 Hz to 20 kHz, as sound. Ultrasound refers to frequencies above 20 kHz, while infrasound represents frequencies below 20 Hz. Animal species have varying hearing ranges.

Speed of sound: The speed of sound is the distance sound travels per unit of time in an elastic medium. In air at 20°C (68°F), it is about 343 m/s or one km in 2.91 s. The speed changes with temperature and medium. As a simple explanation, it is how fast vibrations travel.

Ultrasound: Ultrasound is high-frequency sound beyond the range of human hearing, typically above 20 kilohertz. It encompasses the principles of acoustic waves, extending from 20 kHz to gigahertz frequencies.

Amplitude: Amplitude refers to the measure of change in a periodic variable within a single period. It also represents the magnitude of a non-periodic signal compared to a reference value. The concept of amplitude involves different definitions based on the magnitude of differences between extreme values. In older references, the phase of a periodic function may be referred to as amplitude.

Frequency: Frequency is the number of times a repeating event occurs in a given time unit, typically measured in hertz. It can also be called temporal frequency to avoid confusion with spatial frequency. The period is the time interval between events, with the relationship between period and frequency being reciprocal.

Wavelength: Wavelength is the distance over which a wave repeats its shape. It is the distance between consecutive points of the same phase on the wave. It is a characteristic of both traveling and standing waves. The inverse of wavelength is spatial frequency. The Greek letter lambda (λ) is commonly used to represent wavelength. The term can also apply to modulated waves and sinusoidal envelopes formed by interference.

Diffraction: Diffraction refers to the interference or bending of waves around an obstacle or through an aperture. The obstacle or aperture becomes a secondary source of the wave. It was first observed by Italian scientist Francesco Maria Grimaldi in 1660.

Doppler effect: The Doppler effect is the change in wave frequency based on the movement of an observer relative to the wave source. It was first described by physicist Christian Doppler in 1842. A common example is the change in pitch heard when a vehicle with a horn approaches and moves away from an observer. The frequency is higher when approaching, the same when passing, and lower when receding.

Wave interference: Wave interference is a phenomenon in physics where two coherent waves combine, resulting in a wave with either higher intensity or lower amplitude. It occurs with various types of waves, such as light, radio, sound, and water waves, as well as in electrical waves in loudspeakers.

Noise: Noise is undesired, disruptive sound that is loud and unpleasant to hear. It is indistinguishable from desired sound in terms of physics, as both are vibrations in a medium. However, the brain's perception of a sound determines whether it is classified as noise.

Resonance: Resonance is the phenomenon where an object or system absorbs energy and vibrates with greater amplitude when exposed to an external force or vibration that matches its natural frequency. It can occur in mechanical, electrical, or acoustic systems and has both beneficial applications, like in musical instruments and radio receivers, as well as harmful effects, such as excessive vibrations or structural failure.

Superposition principle: The superposition principle states that in linear systems, the combined response to multiple stimuli is the sum of their individual responses. For example, if input A produces response X and input B produces response Y, then the combined input (A + B) produces response (X + Y).

Wave equation: The wave equation is a fundamental equation in physics that describes various types of waves, such as mechanical and electromagnetic waves. It is a second-order linear partial differential equation and has applications in acoustics, electromagnetism, and fluid dynamics.