Babylon NG
Simply the best definition!

Download it's free

absorptivity (extinction coefficient; absorption cross section; decadic absorptivity)

General Chemistry Glossary

Download this dictionary
absorptivity (extinction coefficient; absorption cross section; decadic absorptivity)
 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.