molar absorptivity

The molar extinction coefficient, also known as molar absorptivity, is a measure of how strongly a chemical species at a given wavelength absorbs light at that wavelength. It is an intrinsic property of the species; the actual absorbance, A, of a sample is dependent on the pathlength l and the concentration c of the species via the Beer-Lambert law, . The units of ε are usually in mol L-1cm-1 or M cm-1.
See more at Wikipedia.org...

() Compare with absorptivity and absorbance .The absorbance per centimeter of path length when the concentration of absorbing material is 1 M; = A/(bc) where a, A, b, and c are the molar absorptivity, absorbance, path length in cm, and concentration in mol/L, respectively. Molar absorptivities are sometimes written as absorption cross sections by expressing the concentration in units of mol/cm3, so that the molar absorptivity takes on units of cm2/mol.