Scientific Definitions

Other pages may contain things that could be considered as definitions. In particular: for definitions of sub–atomic particles, see Particles.

The Electromagnetic Spectrum

In ascending order of wavelength, and descending order of frequency:

Shortest wavelength (highest frequency)		Gamma rays
		X–rays
		Ultra–violet
Shortest wavelength in the visible spectrum		Violet (or blue)
		Visible light
Longest wavelength in the visible spectrum		Red
		Infra–red
		Microwave
		Radio waves
Longest wavelength (lowest frequency)		Electrical energy

Other

Q: What is defined as / What name is given to ... ?		A:
Rate of change of velocity (either speed or direction) over time		Acceleration
The group of nitrogenous chemicals, found in plants, that includes nicotine, caffeine, morphine, strychnine and quinine		Alkaloids
Different forms of the same element, with different chemical bond structures between atoms and different physical characteristics. For example: graphite, charcoal and diamond are allotropes of carbon		Allotrope
A mixture of two or more chemical elements, at least one of which is a metal		Alloy
The positive electrode in an electric cell		Anode
Material that's (theoretically) produced in a nuclear reaction when the charges of protons and electrons are reversed		Antimatter
The coil of an electric motor or dynamo		Armature
The number of atoms, molecules or elementary particles in a mole of any substance (slightly over 6 * 1023; a mole is defined as this number of such entities)		Avogadro's Number
The weight of a pendulum		Bob
The continuous movement of molecules in a fluid		Brownian motion
The upward force exerted by a fluid that opposes the weight of an immersed object (a.k.a. upthrust)		Buoyancy
The quantity of heat required to heat one gram of water by one degree Kelvin		Calorie
The ability to store an electric charge		Capacitance
A substance that speeds up a chemical reaction but is itself unchanged		Catalyst
The negative electrode in an electric cell		Cathode
The force that appears to act on a body moving in a circular path		Centrifugal
The force that's required to keep a body moving in a circular path (the opposite of centrifugal force)		Centripetal
The inter–molecular force that holds solids and liquids together		Cohesion
A suspension of finely divided particles in a continuous medium from which the particles do not settle out rapidly and are not readily filtered		Colloid
The rapid oxidation of a material in an exothermic chemical process, releasing heat, light, and various reaction products		Combustion
The transfer of heat through a solid		Conduction
The transfer of heat in a liquid or gas by actual movement of the fluid		Convection
The quantity of electricity that's transferred by a current of one ampere in one second		Coulomb
The bond that's formed when two atoms share electrons		Covalent bond
The angle at which total internal reflection occurs		Critical angle
The smallest amount of a particular fissile material that's needed for a sustained nuclear chain reaction		Critical mass
The temperature above which a mineral loses its magnetic properties		Curie point
The formation of an aqueous solution (of a salt) by absorption or adsorption of water from the surrounding environment – cf. Hygroscopy		Deliquescence
Mass per unit of volume (of a body or substance)		Density
The temperature to which air must be cooled to become saturated with water vapor, and below which the airborne water vapour will condense to form liquid water (dew); or (in technical terms), the temperature at which the water vapor in a sample of air at constant barometric pressure condenses into liquid water at the same rate at which it evaporates		Dew point
The bending, spreading and interference of waves emerging through an aperture		Diffraction
A device that allows electricity to pass one way only		Diode
The apparent change in frequency of a wave (sound or electromagnetic) due to relative motion between source and observer – named after a 19th century Austrian physicist; examples are the change in note of a passing vehicle, or the Red Shift in astronomy		Döppler Effect doppler
The migration of a salt to the surface of a porous material, where it forms a coating – either due to invasion by water which dissolves an internally held salt (primary), or after the salt has been carried into the material in a solution (secondary) – sometimes regarded as the opposite of deliquesence		Efflorescence
The tendency of a body to return to its original shape after being subjected to stress		Elasticity
The breaking up of chemical compounds by an electric current		Electrolysis
A solution (or a molten substance) that conducts electricity, or a compound that dissolves in water to form such a solution		Electrolyte
A mixture of two immiscible liquids, one dispersed in the other		Emulsion
The amount of energy in a thermodynamic system that is not available to do work – commonly associated with a state of disorder, randomness, or uncertainty		Entropy
A state in which opposing forces neutralise each other		Equilibrium
A chemical reaction that absorbs heat is said to be		Endothermic
A chemical reaction that gives off heat is said to be		Exothermic
The point about which a lever turns		Fulcrum
The ratio between two numbers A and B, where B is the larger, and A:B = B:A+B (value slightly over 1.618)		Golden ratio
The force that causes things to move towards one another; on Earth it makes things appear to fall towards the Earth		Gravity
The time taken for half of a radioactive isotope's atoms to decay		Half life
The absorption or adsorption of water from the surrounding environment (by a chemical substance) – cf. Deliquescence		Hygroscopy
Space that has more than three dimensions		Hyperspace
Opposition in an electrical circuit to the flow of alternating current (symbol Z)		Impedance
The phenomenon whereby some substances glow in the dark		Incandescence
The property of a body that causes it to resist changes in its velocity		Inertia
A material in which electric current does not flow freely		Insulator
The energy density (energy per unit volume) of a wave (sound or light), multiplied by its velocity – measured as power per unit area		Intensity
The electrically charged particle that's formed when an atom or molecule gains or loses one or more electrons		Ion
Molecules with the same chemical formulae, but different structural arrangement of atoms and therefore different physical and/or chemical properties		Isomers
Atoms with the same number of protons (i.e. atoms of the same element) but different numbers of neutrons, and therefore different atomic masses		Isotopes
The energy that a body possesses by virtue of its motion		Kinetic energy
The heat required to change from solid to liquid with no change in temperature		Latent heat
The illumination produced when an oxy–hydrogen flame is directed at a cylinder of calcium oxide		Limelight
The property of a metal that indicates its ability to be worked, hammered or shaped, by pressure or blows, without breaking		Malleability
The average distance that a gas molecule travels between collisions with others (abbreviated to MFP)		Mean Free Path
The curved surface of a liquid (esp. in a container with vertical walls)		Meniscus
Elements that form positive ions		Metals
The smallest unit of a compound that can exist independently; name comes from the diminutive of the Latin word for "mass"		Molecule
The property of a body that causes it to resist changes in its rotational velocity		Moment of inertia
The quantity of motion in a body, measured as mass times (linear) velocity		(Linear) momentum
A very small and penetrating, uncharged elemental particle – emitted by the sun, passes straight through the Earth		Neutrino
The coloured rings around the point of contact between plane and convex surfaces – caused by light interference		Newton’s rings
The movement of a solvent from a weak solution to a strong, through a semi–permeable membrane		Osmosis
The apparent change in position of an object that results from a change of position of the observer		Parallax
Anything that can cause disease in an animal or plant (e.g. a virus or bacterium) – commonly known as a germ; from the Greek for 'producer of suffering'		Pathogen
The time interval for a complete swing of a pendulum (or any SHM)		Period
The hypothetic substance that was believed to be responsible for burning and rusting, before the discovery of oxygen		Phlogiston
A particle of light (according to Einstein’s theory)		Photon
A gas whose atoms have been completely ionised		Plasma
The opposite of a vacuum (a space entirely filled with matter)		Plenum
A natural or synthetic substance composed of very large molecules that are multiples of simpler chemical units		Polymer
The rate at which work is done by or against a force		Power
A natural number that has no positive divisors other than itself and one		Prime number
The emission and propagation of energy, through space or through a material medium, in the form of waves		Radiation
The imaginary part of electrical impedance: opposition to a (sinusoidal) alternating current		Reactance
A substance with characteristic reactions, used in a chemical test		Reagent
The removal of oxygen from a compound (the reverse of oxidation)		Reduction
The change in direction ('bending') of a wave (e.g. light) on passing from one medium to another		Refraction
Describes the extent to which light is 'bent' when entering a material from a vacuum; defined as the relative speed of light in a vacuum, as compared to in the material; also equal to the sine of the angle of incidence divided by the sine of the angle of refraction		Refractive index
The property that measures the degree to which a body opposes the passage of electrical current through it		Resistance
A solution containing the maximum solute for equilibrium		Saturated solution
A variable that has magnitude but not direction (cf. Vector)		Scalar
The process of converting calcium oxide to calcium hydroxide by adding water		Slaking
The solid dissolved in a liquid, in a solution		Solute
The liquid in which a solid is dissolved, in a solution		Solvent
The relative density of a solid or liquid to water, or of a gas to hydrogen		Specific gravity
The amount of heat required to increase the temperature of unit mass of a substance by one unit (under specified conditions)		Specific heat
Distance travelled divided by time (or per unit of time)		Speed or velocity
The property that indicates the angular momentum of a particle (in nuclear physics)		Spin
The shortest distance between two points		Straight line
Change in length divided by original length		Strain
Force or load per unit area		Stress
Transition of a substance from a solid to a gas, without a liquid phase		Sublimation
A material with electrical resistance close to zero at very low temperatures		Superconductor
The constant velocity reached by a body falling under gravity through a fluid		Terminal velocity
The property of certain gels to become less viscous when stirred or shaken (used in paints)		Thixotropy
The turning moment produced about an axis by a force acting at right angles to a radius from the axis		Torque
A semiconductor device used to amplify and switch electronic signals and electrical power – usually having three or more terminals or electrodes		Transistor
The temperature and pressure at which a substance can coexist in its gas, liquid, and solid phases, in thermodynamic equilibrium		Triple point
The scattering of light by particles in its path (e.g. dust in sunlight)		Tyndall effect
Space devoid of matter		Vacuum
The combining power of an element, especially in relation to the number of hydrogen atoms it can displace or combine with		Valency
A variable that has direction as well as magnitude (cf. Scalar)		Vector
Distance traveled per unit of time, in a specified direction		Velocity
The property of a fluid that makes it resist relative motion of different parts of the fluid (internal motion)		Viscosity
Light containing all visible wavelengths at approximately equal intensities		White light
Sound containing all audible wavelengths at approximately equal intensities		White noise

© Haydn Thompson 2017–23