Absolute Zero - The temperature that is zero on the Kelvin or Rankine temperature scales, also the temperature at which a material is at its lowest energy state.

Absorptivity,  a [Absorptance] - The proportion (as a fraction of 1) of the radiant energy impinging on a material’s surface that is absorbed into the material. For a blackbody, this is unity (1.0). Technically, absorptivity is the internal absorptance  per unit path length. In thermography, the two terms are often used interchangeably.

Accuracy [of Measurement] - The maximum deviation, expressed in % of scale or in degrees Celsius or Fahrenheit, that the reading of an instrument will deviate from an acceptable standard reference.

Ambient Operating Range - Range of ambient temperatures at which an instrument is designed to operate within published performance specifications.

Ambient Temperature - Temperature of the air in the vicinity of the target (target ambient) or the instrument (instrument ambient).

Ambient Temperature Compensation - Correction built into an instrument to provide automatic compensation in the measurement for variations in instrument ambient temperature.

Anomaly - Any irregularity, such as a thermal anomaly on an otherwise isothermal surface. Any indication that deviates from what is expected.

Apparent Temperature - The target surface temperature indicated by an infrared point sensor, line scanner or imager before temperature corrections are made.

Artifact - A product of artificial character due to extraneous agency; an error caused by an uncompensated anomaly. In thermography, an emissivity artifact simulates a change in surface temperature but is not a real change. A hot solar reflection or a cold reflection due to narcissus would be examples of artifacts.

Atmospheric Windows [Infrared] - The spectral intervals within the infrared spectrum in which the atmosphere transmits radiant energy well (atmospheric absorption is a minimum.). These are roughly defined as 3-5 µm and 8-14 µm.