A measure of the rate of decrease in the intensity of electromagnetic radiation (as light) as it passes through a given substance

• The fraction of incident radiant energy absorbed per unit mass or thickness of an absorber

Wiktionary Word definitions in Wiktionary
n. (context optics physics English) Absorbed radiation and incident radiation in a ratio format; a measurement that shows how well a surface absorbs radiation. (First attested in the mid 20 th century.) (R:SOED5: page=9)

WordNet Word definitions in WordNet
n. a measure of the rate of decrease in the intensity of electromagnetic radiation (as light) as it passes through a given substance; the fraction of incident radiant energy absorbed per unit mass or thickness of an absorber; "absorptance equals 1 minus ...

Wikipedia Word definitions in Wikipedia
Absorptance of the surface of a material is its effectiveness in absorbing radiant energy . It is the fraction of incident electromagnetic power that is absorbed at an interface, in contrast to the absorption coefficient , which is the ratio of the absorbed ...