absorption cross-section, σ in biology

https://dx.doi.org/10.1351/goldbook.A00038
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:
See also:
PAC, 1990, 62, 2167 (Glossary of atmospheric chemistry terms (Recommendations 1990)) on page 2169
PAC, 1996, 68, 2223 (Glossary of terms used in photochemistry (IUPAC Recommendations 1996)) on page 2226