Analogous to absorption coefficient but taking into account also the effects due to scattering and luminescence. It was formerly called extinction coefficient.
Source:
Green Book, 2nd ed., p. 32 (https://goldbook.iupac.org/files/pdf/green_book_2ed.pdf)
See also:
Orange Book, 2nd ed., p. 212 (https://media.iupac.org/publications/analytical_compendium/)
PAC, 1996, 68, 957. 'Glossary of terms in quantities and units in Clinical Chemistry (IUPAC-IFCC Recommendations 1996)' on page 963 (https://doi.org/10.1351/pac199668040957)