# Glossary of physics

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This glossary of physics terms is a list of definitions about physics, its sub-disciplines, and related fields.

## B

• Babinet's principle – A theorem concerning diffraction that states that the diffraction pattern from an opaque body is identical to that from a hole of the same size and shape except for the overall forward beam intensity.
• Background radiation – The ubiquitous ionising radiation that the general population is exposed to.
• Balmer series
• Barometer – A scientific instrument used in meteorology to measure atmospheric pressure. Pressure tendency can forecast short term changes in the weather.
• Base unit
• Battery –Battery is combination of two or more cells (electric), which produces electricity.
• Beam – A structural element that is capable of withstanding load primarily by resisting bending.Beams are traditionally descriptions of building or civil engineering structural elements, but smaller structures such as truck or automobile frames, machine frames, and other mechanical or structural systems contain beam structures that are designed and analyzed in a similar fashion.
• Bending – (also known as flexure) The behavior of a slender structural element subjected to an external load applied perpendicularly to a longitudinal axis of the element.
• Bending moment
• Bernoulli equation
• Bernoulli's principle
• Bessel function
• Beta particle – High-energy, high-speed electrons or positrons emitted by certain types of radioactive nuclei.
• Big Bang – The prevailing cosmological model that describes the early development of the Universe.
• Binary star – A binary star is a star system consisting of two stars orbiting around their common centre of mass.
• Binding energy – The mechanical energy required to disassemble a whole into separate parts. A bound system typically has a lower potential energy than the sum of its constituent parts.
• Binomial random variable
• Biocatalysis
• Biophysics – An interdisciplinary science using methods of, and theories from, physics to study biological systems
• Black hole – A region of space-time where gravity prevents anything, including light, from escaping.
• Black-body radiation – The type of electromagnetic radiation within or surrounding a body in thermodynamic equilibrium with its environment, or emitted by a black body (an opaque and non-reflective body) held at constant, uniform temperature. The radiation has a specific spectrum and intensity that depends only on the temperature of the body.
• Block and tackle – A system of two or more pulleys with a rope or cable threaded between them, usually used to lift or pull heavy loads.
• Bohr model
• Boiling point – The temperature at which a liquid is changed into a gas; the vapour pressure of liquid and gas are equal at this temperature
• Boiling point elevation
• Boltzmann constant – is a physical constant relating the average kinetic energy of particles in a gas with the temperature of the gas. It is the gas constant R divided by the Avogadro constant NA:
• Bose-Einstein condensate
• Boson – One of two classes of elementary particles, the other being fermions. An important characteristic of bosons is that there is no limit to the number that can occupy the same quantum state.
• Boyle's law – The volume of a given mass of a gas at constant temperature is inversely proportional to the pressure
• Bra–ket notation
• Bragg's law
• Bremsstrahlung – Radiation emitted by the acceleration of unbound charged particles
• Brewster's angle
• British thermal unit – A traditional unit of energy equal to about 1055 joules. It is the amount of energy needed to heat one pound of water by one degree Fahrenheit. In scientific contexts the Btu has largely been replaced by the SI unit of energy, the joule.
• Brittle – A material is brittle if, when subjected to stress, it breaks without significant Brittle materials absorb relatively little energy prior to fracture, even those of high strength. Breaking is often accompanied by a snapping sound.
• Brownian motion – The presumably random moving of particles suspended in a fluid (liquid or gas) resulting from their bombardment by the fast-moving atoms or molecules in the gas or liquid. Also called pedesis
• Bulk modulus – A measure of a substance's resistance to uniform compression. It is defined as the ratio of the infinitesimal pressure increase to the resulting relative decrease of the volume. Its base unit is the pascal.
• Buoyancy – An upward force exerted by a fluid, that opposes the weight of an immersed object.

## D

• Dalton's law
• Damped vibration
• Damping
• Darcy–Weisbach equation
• Dark energy
• Dark matter
• DC motor – A mechanically commutated electric motor powered from direct current (DC).
• Decibel
• Definite integral
• Deflection – The degree to which a structural element is displaced under a load. It may refer to an angle or a distance.
• Deformation (engineering)
• Deformation (mechanics)
• Density – The mass density or density of a material is its mass per unit volume. Mathematically, density is defined as mass divided by volume.
• Derivative
• Derived unit
• Destructive interference
• Dew point
• Dielectric
• Diffraction
• Dispersion
• Displacement (fluid) – Occurs when an object is immersed in a fluid, pushing it out of the way and taking its place. The volume of the fluid displaced can then be measured, and from this the volume of the immersed object can be deduced (the volume of the immersed object will be exactly equal to the volume of the displaced fluid).
• Displacement (vector) – The shortest distance from the initial to the final position of a point. Thus, it is the length of an imaginary straight path, typically distinct from the path actually travelled by.
• Distance – A numerical description of how far apart objects are.
• Drift velocity
• Doppler effect – The change in frequency of a wave (or other periodic event) for an observer moving relative to its source.The received frequency is higher (compared to the emitted frequency) during the approach, it is identical at the instant of passing by, and it is lower during the recession.
• Drag – Forces which act on a solid object in the direction of the relative fluid flow velocity. Unlike other resistive forces, such as dry friction, which is nearly independent of velocity, drag forces depend on velocity.
• Ductility – A solid material's ability to deform under tensile stress; this is often characterized by the material's ability to be stretched into a wire.
• Dynamics
• Dyne

## G

• Gamma ray – Electromagnetic radiation of high frequency and therefore high energy.
• Gamma-ray burst
• General relativity
• Geophysics
• Gluon
• Graham's law of diffusion
• Gravitation – (or Gravity) A natural phenomenon by which physical bodies attract each other with a force proportional to their masses.
• Gravitational constant – (denoted by G) A physical constant involved in the calculation(s) of gravitational force between two bodies. (Also known as 'the universal gravitational constant', or 'Newton's constant')
• Gravitational energy – The potential energy associated with the gravitational field.
• Gravitational field – A model used to explain the influence that a massive body extends into the space around itself, producing a force on another massive body. Thus, a gravitational field is used to explain gravitational phenomena, and is measured in newtons per kilogram (N/kg).
• Gravitational potential – The gravitational potential at a location is equal to the work (energy transferred) per unit mass that is done by the force of gravity to move an object to a fixed reference location.
• Gravitational waves -Gravitational waves are ripples in the curvature of spacetime that propagate as waves, generated in certain gravitational interactions and travelling outward from their source.
• Gravity – (or gravitation) A natural phenomenon by which physical bodies attract each other with a force proportional to their masses.
• Ground
• Ground reaction force
• Ground state
• Group velocity

## I

• Ice point – A physical process that results in the phase transition of a substance from a liquid to a solid.
• Impedance – The measure of the opposition that a circuit presents to a current when a voltage is applied.
• Indefinite integral
• Inductance
• Infrasound
• Inertia – The resistance of any physical object to a change in its state of motion or rest, or the tendency of an object to resist any change in its motion.
• Inductive reactance
• Integral
• Integral transform
• International System of Units – The International System of Units (abbreviated SI) is the modern form of the metric system. It comprises a system of units of measurement devised around seven base units and the convenience of the number ten.
• Invariant mass
• Ion – An atom or molecule in which the total number of electrons is not equal to the total number of protons, giving the atom a net positive or negative electrical charge.
• Ionic bond – A type of chemical bond formed through an electrostatic attraction between two oppositely charged ions.
• Ionization – The process of converting an atom or molecule into an ion by adding or removing charged particles such as electrons or ions.
• Isotope – Variants of a particular chemical element. While all isotopes of a given element share the same number of protons, each isotope differs from the others in its number of neutrons.

## J

• Joule – A derived unit of energy, work, or amount of heat in the International System of Units.

## K

• Kelvin – A unit of measurement for temperature. The Kelvin scale is an absolute, thermodynamic temperature scale using as its null point absolute zero.
• Kinematics – The branch of classical mechanics that describes the motion of points, bodies (objects) and systems of bodies (groups of objects) without consideration of the causes of motion. The study of kinematics is often referred to as the geometry of motion.
• Kirchhoff's circuit laws – Two approximate equalities that deal with the current and voltage in electrical circuits.also called Kirchhoff's rules or simply Kirchhoff's laws (see also Kirchhoff's laws for other meanings of that term).
• Kirchhoff's equations – In fluid dynamics, the Kirchhoff equations describe the motion of a rigid body in an ideal fluid.

## L

• Laminar flow – (or streamline flow) Occurs when a fluid flows in parallel layers, with no disruption between the layers.
• Laplace transform
• Laplace–Runge–Lenz vector – (or LRL vector) A vector used chiefly to describe the shape and orientation of the orbit of one astronomical body around another, such as a planet revolving around a star. For two bodies interacting by Newtonian gravity, the LRL vector is a constant of motion, meaning that it is the same no matter where it is calculated on the orbit; equivalently, the LRL vector is said to be conserved.
• Laser
• Law of universal gravitation
• LC circuit
• Lenz's law
• Lepton – An elementary particle which does not undergo strong interactions, but is subject to the Pauli exclusion principle. Two main classes of leptons exist: charged leptons (also known as the electron-like leptons), and neutral leptons (better known as neutrinos).
• Lever – A machine consisting of a beam or rigid rod pivoted at a fixed hinge, or fulcrum.
• Light – Visible light (commonly referred to simply as light) is electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight.
• Linear actuator-- A form of motor that generates a linear movement directly.
• Linear algebra – The branch of mathematics concerning vector spaces, often finite or countably infinite dimensional, as well as linear mappings between such spaces.
• Line of force
• Linear elasticity – The mathematical study of how solid objects deform and become internally stressed due to prescribed loading conditions. Linear elasticity is a simplification of the more general nonlinear theory of elasticity and is a branch of continuum mechanics.
• Liouville's Theorem - Phase space volume is conserved.
• Liquid – A classical state of matter with a definite volume but no fixed shape.
• Liquid crystal – (LCs) A matter in a state that has properties between those of conventional liquid and those of solid crystal. For instance, an LC may flow like a liquid, but its molecules may be oriented in a crystal-like way.
• Longitudinal wave

## M

• M-theory – An extension of string theory in which 11 dimensions are identified.
• Mach number – A dimensionless quantity representing the ratio of speed of an object moving through a fluid and the local speed of sound.
• Machine – A powered tool consisting of one or more parts that is constructed to achieve a particular goal. Machines are usually powered by mechanical, chemical, thermal or electrical means, and are frequently motorised.
• Machine element – Refers to an elementary component of a machine. consists of three basic types: structural components. mechanisms and control components
• Maclaurin series – A representation of a function as an infinite sum of terms that are calculated from the values of the function's derivatives at a single point.
• Magnetic field – A mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude (or strength); as such it is a vector field.
• Magnetism – A property of materials that respond to an applied magnetic field.
• Mass balance – An application of conservation of mass to the analysis of physical systems, also called 'material balance'.
• Mass density – A materials mass per unit volume, also just called density.
• Mass flux – Is the rate of mass flow per unit area. The common symbols are j, J, φ, or Φ, sometimes with subscript m to indicate mass is the flowing quantity. Its SI units are kg s−1 m−2.
• Mass moment of inertia – A property of a distribution of mass in space that measures its resistance to rotational acceleration about an axis.
• Mass number – The total number of protons and neutrons (together known as nucleons) in an atomic nucleus, also called atomic mass number or nucleon number.
• Mass spectrometry
• Material properties
• Materials science
• Mathematical physics – "The application of mathematics to problems in physics and the development of mathematical methods suitable for such applications and for the formulation of physical theories."
• Mathematics – The abstract study of topics encompassing quantity, structure, space, change, and other properties; it has no generally accepted definition.
• Matrix – A rectangular array of numbers, symbols, or expressions, arranged in rows and columns. The individual items in a matrix are called its elements or entries.
• Matter – Generally considered to be a substance (often a particle) that has rest mass and (usually) also volume.
• Maxwell's equations – A set of partial differential equations that, together with the Lorentz force law, form the foundation of classical electrodynamics, classical optics, and electric circuits.Maxwell's equations describe how electric and magnetic fields are generated and altered by each other and by charges and currents.
• Mean – For a data set, the arithmetic mean is equal to the sum of the values divided by the number of values.
• Measures of central tendency – A term which relates to the way in which quantitative data tend to cluster around some value. A measure of central tendency is any of a number of ways of specifying this "central value."
• Mechanical energy
• Mechanical filter
• Mechanical equilibrium
• Mechanical wave
• Mechanics – The branch of science concerned with the behaviour of physical bodies when subjected to forces or displacements, and the subsequent effects of the bodies on their environment.
• Median – In statistics and probability theory; the median of a finite list of numbers can be found by arranging all the observations from lowest value to highest value and picking the middle one. If there is an even number of observations the median is then usually defined to be the mean of the two middle values.
• Melting – (or fusion) A physical process that results in the phase transition of a substance from a solid to a liquid.
• Meson – Hadronic subatomic particles composed of one quark and one antiquark, bound together by the strong interaction.All mesons are unstable, with the longest-lived lasting for only a few hundredths of a microsecond.
• Mode – The value that appears most often in a set of data.
• Modulus of elasticity – The mathematical description of an object or substance's tendency to be deformed elastically (i.e., non-permanently) when a force is applied to it. The elastic modulus of an object is defined as the slope of its stress–strain curve in the elastic deformation region. As such, a stiffer material will have a higher elastic modulus.
• Molality
• Molarity
• Molar concentration
• Molar mass – A physical property of matter. It is defined as the mass of a given substance divided by its amount of substance. The unit for molar mass is g/mol.
• Molecule – An electrically neutral group of two or more atoms held together by covalent chemical bonds. Molecules are distinguished from ions by their lack of electrical charge.
• Molecular physics – The study of the physical properties of molecules, the chemical bonds between atoms as well as the molecular dynamics.closely related to atomic physics and overlaps greatly with theoretical chemistry, physical chemistry and chemical physics.
• Moment
• Moment of inertia – A property of a distribution of mass in space that measures its resistance to rotational acceleration about an axis.
• Monochromatic light
• Muon – An elementary particle similar to the electron, with unitary negative electric charge (−1) and a spin of 1⁄2. Classified as a lepton. Not believed to have any sub-structure

## N

• Nanoengineering – The practice of engineering on the nanoscale. Nanoengineering is largely a synonym for nanotechnology, but emphasizes the engineering rather than the pure science aspects of the field.
• Nanotechnology – (sometimes shortened to "nanotech") The manipulation of matter on an atomic and molecular scale. A more generalized description of nanotechnology is 'the manipulation of matter with at least one dimension sized from 1 to 100 nanometers'.
• Navier–Stokes equations
• Neurophysics
• Neutrino – An electrically neutral subatomic particle. The neutrino is denoted by the Greek letter ν (nu). All evidence suggests that neutrinos have mass but that their mass is tiny even by the standards of subatomic particles. Their mass has never been measured accurately.
• Newton's laws of motion – The three physical laws that form the basis for classical mechanics. They describe the relationship between the forces acting on a body and its motion due to those forces.
• Newton's law of universal gravitation
• Newtonian fluid
• Normal force
• nth root
• Nuclear physics – The field of physics that studies the constituents and interactions of atomic nuclei.
• Nuclear transmutation

## O

• Ohm – The SI derived unit of electrical resistance.
• Ohm's law – States that the current through a conductor between two points is directly proportional to the potential difference across the two points.
• Optical tweezers - an optomechanical device used for the capture, analysis and manipulation of dielectric objects or particles which operates via the application of force by the electric field of light.
• Optically detected magnetic resonance – an optical technique for the initialisation and readout of quantum spin in some crystal defects.
• Optics – The branch of physics which involves the behaviour and properties of light, including its interactions with matter and the construction of instruments that use or detect it. Optics usually describes the behaviour of visible, ultraviolet, and infrared light, however, Because light is an electromagnetic wave, other forms of electromagnetic radiation such as X-rays, microwaves, and radio waves exhibit similar properties.

## P

• Parity (mathematics)
• Parity (physics)
• Paraffin
• Parallel circuit
• Particle accelerator
• Particle displacement
• Particle physics – A branch of physics which studies the nature of particles that are the constituents of what is usually referred to as matter and radiation.
• Pascal's law – A principle in fluid mechanics that states that pressure exerted anywhere in a confined incompressible fluid is transmitted equally in all directions throughout the fluid such that the pressure variations (initial differences) remain the same.
• Pendulum
• Periodic table – A tabular display of the chemical elements, organised on the basis of their atomic numbers, electron configurations, and recurring chemical properties. Elements are presented in order of increasing atomic number (number of protons).
• pH – A measure used to describe whether a liquid is acidic or basic (or alkaline).
• Phase (matter)
• Phase (waves)
• Phase equilibrium
• Phosphorescence
• Photon – An elementary particle, the quantum of light and all other forms of electromagnetic radiation, and the force carrier for the electromagnetic force.
• Physical chemistry – The study of macroscopic, atomic, subatomic, and particulate phenomena in chemical systems in terms of laws and concepts of physics.
• Physical quantity
• Physics – The natural science that involves the study of matter and its motion through space and time, along with related concepts such as energy and force. More broadly, it is the general analysis of nature, conducted in order to understand how the universe behaves.
• Planck constant – (denoted h, also called Planck's constant) A physical constant that is the quantum of action in quantum mechanics.
• Plasma
• Plasma physics
• Plasticity
• Pneumatics – The control of mechanical force and movement, generated by the application of compressed gas.
• Positron
• Power (electric) – The rate at which electric energy is transferred by an electric circuit.
• Power (physics)
• Pressure – The ratio of force to the area over which that force is distributed.
• Probability – A measure of the expectation that an event will occur or a statement is true. Probabilities are given a value between 0 (will not occur) and 1 (will occur). The higher the probability of an event, the more certain we are that the event will occur.
• Probability distribution
• Probability theory
• Proton
• Psi particle
• Pulley – A wheel on an axle that is designed to support movement of a cable or belt along its circumference. Pulleys are used in a variety of ways to lift loads, apply forces, and to transmit power.
• Pulse
• Pulse wave

## R

• Redshift – Redshift happens when light seen coming from an object that is moving away is proportionally increased in wavelength, or shifted to the red end of the spectrum.
• Refraction
• Refractive index
• Relative atomic mass
• Rigid body – An idealization of a solid body in which deformation is neglected. In other words, the distance between any two given points of a rigid body remains constant in time regardless of external forces exerted on it. Even though such an object cannot physically exist due to relativity, objects can normally be assumed to be perfectly rigid if they are not moving near the speed of light.
• Roche limit – (also Roche radius) The distance within which a celestial body, held together only by its own gravity, will disintegrate due to a second celestial body's tidal forces exceeding the first body's gravitational self-attraction. Inside the Roche limit, orbiting material will tend to disperse and form rings, while outside the limit, material will tend to coalesce.
• Rotational energy – (or angular kinetic energy) The kinetic energy due to the rotation of an object and forms part of its total kinetic energy.
• Rotational speed – (also speed of revolution) The number of complete rotations or revolutions a rotating body makes per time unit
• Rydberg formula – A formula used in atomic physics to describe the wavelengths of spectral lines of many chemical elements.

## S

• Scalar (mathematics)
• Scalar (physics) – A scalar is a simple physical quantity that is unchanged by coordinate system rotations or translations (in Newtonian mechanics), or by Lorentz transformations or central-time translations (in relativity). A scalar is a quantity that can be described by a single number, unlike vectors, tensors, etc. which are described by several numbers which describe magnitude and direction.
• Scattering – A general physical process where some forms of radiation, such as light, sound, or moving particles, are forced to deviate from a straight trajectory by one or more localised non-uniformities in the medium through which they pass.
• Science – A systematic enterprise that builds and organises knowledge in the form of testable explanations and predictions about the universe.
• Screw – A mechanism that converts rotational motion to linear motion, and a torque (rotational force) to a linear force.
• Series circuit
• Shear strength
• Shear stress
• Schrödinger equation – Describes the time evolution of wave functions in quantum mechanics
• SI units – (abbreviated SI) The modern form of the metric system. It comprises a system of units of measurement devised around seven base units and the convenience of the number ten.
• Simple harmonic motion
• Simple machine – A mechanical device that changes the direction or magnitude of a force.[2] In general, they can be defined as the simplest mechanisms that provide mechanical advantage (also called leverage).
• Siphon – A tube in an inverted U shape that causes a liquid to flow uphill without pumps, powered by the fall of the liquid as it flows down the tube under the pull of gravity. May more generally refer to a wide variety of devices that involve the flow of liquids through tubes.
• Snell's law
• Solar cell
• Solid mechanics
• Solid-state physics
• Solubility – The property of a solid, liquid, or gaseous chemical substance called solute to dissolve in a solid, liquid, or gaseous solvent to form a homogeneous solution of the solute in the solvent. The solubility of a substance fundamentally depends on the used solvent as well as on temperature and pressure.
• Sound – A mechanical wave that is an oscillation of pressure transmitted through a solid, liquid, or gas, composed of frequencies within the range of hearing.
• Special relativity
• Speed of light – The speed of light in vacuum, commonly denoted c, is a universal physical constant important in many areas of physics. Its value is 299,792,458 metres per second, a figure that is exact because the length of the metre is defined from this constant and the international standard for time.
• Spherical aberration
• Spin quantum number
• Standard atomic weight
• Standard Model
• Standing wave
• State of matter
• Statics – The branch of mechanics that is concerned with the analysis of loads (force and torque, or "moment") on physical systems in static equilibrium, that is, in a state where the relative positions of subsystems do not vary over time, or where components and structures are at a constant velocity.
• Statistics – The study of the collection, organisation, analysis, interpretation, and presentation of data. It deals with all aspects of this, including the planning of data collection in terms of the design of surveys and experiments.
• Stiffness – The rigidity of an object — the extent to which it resists deformation in response to an applied force.
• Strain – The transformation of a body from a reference configuration to a current configuration.[1] A configuration is a set containing the positions of all particles of the body.
• Strain hardening
• Strength of materials
• Stress – (1) An applied force or system of forces that tends to strain or deform a body. (2) A measure of the internal forces acting within a deformable body. (3) A quantitative measure of the average force per unit area of a surface within a body on which internal forces act.
• Stress–strain curve
• String duality
• String theory
• Subatomic particle – Subatomic particles are the particles smaller than an atom.
• Sublimation – The process of transformation directly from the solid phase to the gas phase without passing through an intermediate liquid phase. Sublimation is an endothermic phase transition that occurs at temperatures and pressures below a substance's triple point in its phase diagram.
• Superconductivity
• Superconductor – A phenomenon of exactly zero electrical resistance and expulsion of magnetic fields occurring in certain materials when cooled below a characteristic critical temperature.
• Superposition principle
• Superhard material
• Supernova – A stellar explosion that is more energetic than a nova. Supernovae can be triggered in one of two ways: by the sudden reignition of nuclear fusion in a degenerate star; or by the collapse of the core of a massive star.
• Surface tension

## V

• Vacuum – An area of space which contains no matter.
• Valence electron – An electron that is associated with an atom, and that can participate in the formation of a chemical bond
• Valence shell – The outermost electron shell of an atom.
• Variable capacitor
• Variable resistor
• Vector – A quantity that has both magnitude and direction
• Vector space – A mathematical structure formed by a collection of elements called vectors, which may be added together and multiplied ("scaled") by numbers, called scalars in this context
• Velocity
• Virtual image
• Virtual particle
• Viscoelasticity
• Viscosity
• Visible light – A form of electromagnetic (EM) radiation, as are radio waves, infrared radiation, ultraviolet radiation, X-rays and microwaves. Generally,visible light is defined as the wavelengths that arevisible to most human eyes.
• Volta potential
• Voltmeter – An instrument used for measuring electrical potential difference between two points in an electric circuit. Analog voltmeters move a pointer across a scale in proportion to the voltage of the circuit.
• Volt per meter

## W

• Watt – A derived unit of power in the International System of Units (SI). The unit, defined as one joule per second, measures the rate of energy conversion or transfer.
• Wave – A disturbance or oscillation that travels through spacetime, accompanied by a transfer of energy.
• Wave equation
• Wave function
• Wavelength – The wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.
• Weak interaction – Weak interaction (also weak force/weak nuclear force) is one of the four fundamental forces of nature, alongside the strong nuclear force, electromagnetism, and gravitation. It is responsible for the radioactive decay of subatomic particles and initiates the process known as hydrogen fusion in stars.
• Weber
• Wedge – A triangular shaped round tool, a compound and portable inclined plane, and one of the six classical simple machines.
• Wheel and axle – One of six simple machines identified by Renaissance scientists drawing from Greek texts on technology. The wheel and axle is generally considered to be a wheel attached to an axle so that these two parts rotate together in which a force is transferred from one to the other.
• White dwarf – (also degenerate dwarf) A stellar remnant composed mostly of electron-degenerate matter. They are very dense; a white dwarf's mass is comparable to that of the Sun and its volume is comparable to that of the Earth. Its faint luminosity comes from the emission of stored thermal energy.
• Wind – The flow of gases on a large scale.
• Wind shear (sometimes called wind gradient) – A difference in wind speed and direction over a relatively short distance in the atmosphere. Wind shear can be broken down into vertical and horizontal components, with horizontal wind shear seen across fronts and near the coast, and vertical shear typically near the surface, though also at higher levels in the atmosphere.
• Work function

## X

• X ray – a high energy photon (between 100 electron volts (eV) and 100 keV), with a wavelength shorter than that of ultraviolet radiation and longer than that of gamma radiation.

## Z

• Zeeman effect – the effect of splitting a spectral line into several components in the presence of a static magnetic field by the lifting of degeneracy in electronic states.