A

AATCC
American Association of Textile Chemists and Colorists.

Absorption
The convertion of radiant energy (i.e. light) to a different form of energy (i.e. heat). 

Acceptability
Achromatic
A colour perceived as having no hue, that is, as white, black or grey.

Additive primaries
Allen algorithm
A standard algorithm often used by colour match prediction software for calculating the ratios of different colorants (Dyes, Inks, Pigments etc) required to match a reflectance curve.

ANLAB, AN(40)
A colour difference formula devised by

ANSI
American National Standards Institute.
 
Aperture mode
Colour seen through an aperture which prevents its association with a specific object or source.