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: \[\sigma _{\rm{r}}=P_{\rm{r}}\ \pi \ b_{\max }^{2}\] where \(P_{\rm{r}}\) is the reaction probability.