Absolute humidity:  The ratio of water vapor in a sample of air to the volume of the sample. 
Absolute zero:  The temperature of  273.16 or 0 K at which molecular motion vanishes. 
Absorptance:  The ratio of the total absorbed radiation to the total incident radiation. 
Acceleration:  The rate of change of velocity with respect to time. 
Acceleration due to gravity:  The acceleration imparted to bodies by the attractive force of the earth or any other heavenly body. 
Achromatic:  capable of transmitting light without decomposing it into its constituent colors. 
Acoustics:  The science of the production, transmission and effects of sound. 
Acoustic shielding:  A sound barrier that prevents the transmission of acoustic energy. 
Adiabatic:  Any change in which there is no gain or loss of heat. 
Afocal lens:  A lens of zero convergent power, whose focal points are infinitely distant. 
Albedo:  The fraction of the total light incident on a reflecting surface, especially a celestial body, which is reflected back in all directions. 
Alpha particle:  The nucleus of a helium atom (two protons and two neutrons) emitted as radiation from a decaying heavy nucleus. 
Alternating current:  The electric current that changes its direction periodically. 
Amorphous:  Solids which have neither definite form nor structure. 
Ampere:  S.I. Unit of electric current, one ampere is the flow of one coulomb of charge per second. 
Amplitude:  The maximum absolute value attained by the disturbance of a wave or by any quantity that varies periodically. 
Angle of contact:  The angle between tangents to the liquid surface and the solid surface inside the liquid, both the tangents drawn at the point of contact. 
Angle of incidence:  The angle between the incident ray and the normal. 
Angle of reflection:  The angle between the reflected ray and the normal. 
Angle of refraction:  The angle between the refracted ray and the normal. 
Angle of repose:  The angle of inclination of a plane with the horizontal such that a body placed on the plane is at the verge of sliding. 
Angstrom:  A unit of length, 1 = 1010 m. 
Angular momentum:  Also called moment of momentum, it is the cross product of position vector and momentum. 
Angular velocity:  The rate of change of angular displacement with time. 
Annihilation:  A process in which a particle and antiparticle combine and release their rest energies in other particles. 
Antineutrino:  The antiparticle of neutrino, it has zero mass and spin ½. 
Archimedes principle:  A body immersed in a fluid experiences an apparent loss in weight which is equal to the weight of the fluid displaced by the body. 
Atomic mass unit:  It is equal to onetwelfth the mass of C 12 isotope of carbon, 1 amu = 1.66x 1027 Kg. 
Atomic number:  The number of protons in an atomic nucleus. 
Avogadro number:  The number of molecules in a gram molecular weight of a substance, it is equal to 6.02 x 1023. 
Avogadro's law:  Under the same conditions of temperature and pressure, equal volumes of all gases contain equal number of molecules. 
