absorption coefficient, coefficient of absorption, absorptance(noun)

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 transmittance"

absorption coefficient(Noun)

a measure of the absorption of electromagnetic radiation as it passes through a specific substance; the fraction of incident radiation absorbed by unit mass or unit thickness