This term, sometimes used as equivalent to absorbance, is no longer recommended.
See: absorbance, attenuance
Sources:
Green Book, 2nd ed., p. 32 (https://goldbook.iupac.org/files/pdf/green_book_2ed.pdf)
PAC, 1996, 68, 2223. 'Glossary of terms used in photochemistry (IUPAC Recommendations 1996)' on page 2242 (https://doi.org/10.1351/pac199668122223)
PAC, 1996, 68, 957. 'Glossary of terms in quantities and units in Clinical Chemistry (IUPAC-IFCC Recommendations 1996)' on page 973 (https://doi.org/10.1351/pac199668040957)