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

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

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 ...

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 ...