Linear decadic (a,K) and Napierian absorption coefficients (α) are equal to the corresponding absorbances divided by the optical path length through the sample. The molar absorption coefficients (decadic ɛ, Napierian κ) are the linear absorption coefficients divided by the amount concentration.
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)
PAC, 1996, 68, 957. 'Glossary of terms in quantities and units in Clinical Chemistry (IUPAC-IFCC Recommendations 1996)' on page 959 (https://doi.org/10.1351/pac199668040957)