absorptivity

n. quality of being absorptive; capability of absorbing radiation; ratio of radiation absorbed by a body to the amount reflected by the body

In optics, the Beer-Lambert law, also known as Beer's law or the Lambert-Beer law or the Beer-Lambert-Bouguer law (in fact, most of the permutations of these three names appear somewhere in literature) is an empirical relationship that relates the absorption of light to the properties of the material through which the light is traveling.
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Noun
1. (physics) the property of a body that determines the fraction of the incident radiation or sound flux absorbed or absorbable by the body
(synonym) absorption factor
(hypernym) physical property
(classification) physics, physical science, natural philosophy

(n.)
Absorptiveness.
  

 Say it 
(a) Compare with molar absorptivity and absorbance .The absorbance of a solution per unit of path length and per unit concentration; a = A/(bc) where a, A, b, and c are the absorptivity, absorbance, path length, and concentration, respectively. Absorptivity varies with wavelength of the incident light.