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 [Terms] [Book]
See also:
PAC, 1996, 68, 149. (A glossary of terms used in chemical kinetics, including reaction dynamics (IUPAC Recommendations 1996)) on page 158 [Terms] [Paper]