## absorption cross-section, $$\sigma$$

https://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:
Green Book, 2nd ed., p. 32 (https://dev.goldbook.iupac.org/files/pdf/green_book_2ed.pdf)