Molar Napierian absorption coefficient divided by the Avogadro constant. When integrated against the logarithm of wavenumber (or frequency) it is called the integrated absorption cross-section.
Source:
Green Book, 2nd ed., p. 32 (https://goldbook.iupac.org/files/pdf/green_book_2ed.pdf)
See also:
PAC, 1990, 62, 2167. 'Glossary of atmospheric chemistry terms (Recommendations 1990)' on page 2169 (https://doi.org/10.1351/pac199062112167)
PAC, 1996, 68, 2223. 'Glossary of terms used in photochemistry (IUPAC Recommendations 1996)' on page 2226 (https://doi.org/10.1351/pac199668122223)