| 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 = 10-10 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 one-twelfth the mass of C -12 isotope of carbon, 1 amu = 1.66x 10-27 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. |
