molar absorption coefficient, ɛ

Absorbance divided by the absorption pathlength, $l$ , and the amount concentration, $c$ :

$ɛ λ = 1 c l lg ( P λ 0 P λ ) = A λ c l$

where $P λ 0$ and $P λ$ are, respectively, the incident and transmitted spectral radiant power.

Notes:

The term molar absorptivity for molar absorption coefficient should be avoided.
In common usage for $l / cm$ and $c / mol dm −3$ ( $M$ ), $ɛ λ$ results in $dm 3 mol −1 cm −1$ ( $M −1 cm −1$ , the most commonly used unit), which equals $0.1 m 2 mol −1$ (coherent SI units).

Source:

PAC, 2007, 79, 293 (Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006)) on page 371

molar absorption coefficient, $ɛ$