reaction cross-section, σ r

https://dx.doi.org/10.1351/goldbook.R05169
A quantity used in collision theories of reactions in order to interpret calculated or experimental rates. In some collision theories the reaction cross-section is considered to be related to b max, the maximum value of the impact parameter that allows reaction to occur, by:
σ r = P r π b max 2
where P r is the reaction probability.
Source:
PAC, 1996, 68, 149 (A glossary of terms used in chemical kinetics, including reaction dynamics (IUPAC Recommendations 1996)) on page 183