- (analytical chemistry) The constant a in the Beer's law relation A = abc, where A is the absorbance, b the path length, and c the concentration of solution. Also known as absorptive power. Formerly known as absorbency index; absorption constant; extinction coefficient.
- (thermodynamics) The fraction of radiation absorbed by a surface to the total radiation incident on the surface. [First attested in the mid 19th century.]
- The quality of being absorptive; absorptiveness. [First attested in the mid 19th century.]