Aberration: An optical phenomenon resulting from the failure of a lens or mirror to produce a good image.

Absolute Pressure: Actual pressure on a confined gas, irrespective of the atmosphere on the outside.

Absolute Temperature: The temperature measured from absolute zero as in the Kelvin and Rankine scales.

Absolute Zero: The lowest temperature theoretically attainable (at which the kinetic energy of atoms and molecules is minimal.

Absorption: (1) Loss of energy traveling through a medium. (2) Internal taking up of one material by another. (3) Transformation of radiant energy into other forms of energy when passing through a material substance.

Acceleration: Rate of change of velocity.

Accommodation: Changes in focus of the crystalline lens to adjust the eye of various object distances.

Accuracy: (1) The closeness of agreement between a test result and the accepted reference value (ISO 5725-1). (2) Closeness of agreement between the result of measurement and a true value of the measurand. Accuracy is a qualitative concept (VIM:1993)

A/D: Analog-to digital conversion.

Adjustment (of a measuring instrument): The operation of bringing a measuring instrument into a state of performance suitable for its use.

Adsorption: Adhesion of one substance to the surface of another.

Alpha: The current amplification factor when connected in a common base configuration.

Alternating Current (AC): Current which reverses polarity at a uniform frequency.

Altimeter: An instrument that measures the height above ground.

Ambient Temperature: Temperature of the air in the immediate vicinity.

Ammeter: A meter that measures the flow of electrical current in amperes.

Amperes: The basic unit of electric current adopted under the Systeme International d’Unites.

Amplification: The process by which ionization effects are magnified to a degree suitable for their measurement.

Apparent Power: The power value obtained in an alternating current circuit by multiplying the effective values of voltage and current. The result is expressed in volt-amperes, and must be multiplied by the power factor to secure the average or true power in watts.

Arc: A portion of the circumference of a circle.

Atmospheric Pressure: The force exerted on a unit area by the weight of the atmosphere.

Auditee: The organization being audited