collision cross-section, \(\sigma\)

https://doi.org/10.1351/goldbook.C01161
In simple hard sphere collision theory the area of the circle with radius equal to the collision diameter.
Source:
Green Book,2nd ed., p. 56 (https://www.cintegrado.com.br/site/documentos/green_book_2ed.pdf)
See also:
PAC, 1996, 68, 149. 'A glossary of terms used in chemical kinetics, including reaction dynamics (IUPAC Recommendations 1996)' on page 158 (https://doi.org/10.1351/pac199668010149)