(1) A measure of how strongly a chemical species absorbs light at a particular wavelength. It is often expressed in units of cm-1M-1.
(2) A fundamental unit in spectrophotometry.
It is symbolized by ε in Beer-Lambert law, A = εcl, where A is the absorbance of the sample, l is the pathlength (usually in centimeter), and c is the concentration of the of absorbing species in the material (moles per liter).
See also: Beer-Lambert law