The American Heritage® Dictionary of the English Language, 4th Edition
n. The ratio of absorbed to incident radiation.
WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.
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
Word Usage
"Certainly, over the tropical midday deep oceans, the ocean absorptance must be in teh range of 97-98%, since the near normal incident radiation simply goes on down till something absorbs it."