【Absolute index of refraction】
In a transparent material, a number that represents the ratio of the speed of light in vacuum to the speed of light in the material.

【Absolute temperature】
A measure of the average kinetic energy of the molecules in an object; the Kelvin temperature is equal to the Celsius temperature of an object plus 273.16.

【Absolute zero】
A temperature of -273℃, designated as 0 K.

【Absorption spectrum】
A continuous spectrum crossed by dark lines representing absorption of particular wavelengths of radiation by a cooler medium.

【Acceleration】
A vector quantity representing the time rate of change in velocity.

【Activity】
The number of radioactive decays of a radioactive atom per unit of time.

【Adhesion】
The force holding the surfaces of unlike substances together.

【Adiabatic】
In thermodynamics, the process that occurs when no heat is added to or subtracted from a system.

【Alpha decay】
The spontaneous emission of a helium nucleus from certain radioactive atoms.

【Alpha particle】
A helium nucleus ejected from a radioactive atom.

【Alternating current】
An electric current that varies its direction and magnitude according to a regular frequency.

【Ammeter】
A device for measuring the current in an electric circuit when placed in series; a galvanometer with a low-resistance coil placed across it.

【Ampere】
The SI unit of electric current equal to 1 C/s.

【Amplitude】
The maximum displacement of an oscillating particle, medium, or field relative to its rest position.