molar absorption coefficient, \(\varepsilon\)

Also contains definition of: molar decadic absorption coefficient
https://doi.org/10.1351/goldbook.M03972
absorbance divided by the absorption pathlength, l, and the amount concentration, c: \[\varepsilon(\lambda) = \frac{1}{c\, l}\, \text{lg}\! \left ( \frac{P_{\lambda}^{0}}{P_{\lambda}}\right ) = \frac{A(\lambda)}{c\, l}\] where P λ 0 and P λ are, respectively, the incident and transmitted spectral radiant power.
Notes:
  1. The term @[email protected] for molar @[email protected] should be avoided.
  2. In common usage for l/cm and c/mol dm-3 (M), ɛ(λ) results in dm3 mol-1 cm-1 (M-1 cm-1, the most commonly used unit), which equals 0.1 m2 mol-1 (coherent SI units).
Source:
PAC, 2007, 79, 293. 'Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006)' on page 371 (https://doi.org/10.1351/pac200779030293)