absorptance divided by the optical path length. For very low attenuance it approximates the absorption coefficient [within the approximation (1 - e-A) ~ A]. The use of this term is not recommended.
Source:
PAC, 1996, 68, 2223. 'Glossary of terms used in photochemistry (IUPAC Recommendations 1996)' on page 2227 (https://doi.org/10.1351/pac199668122223)