The probability that a reaction occurs when two particles undergo a collision. If N c is the number of collisions occurring in unit time, and N r the number of reactions occurring in unit time: P r = N r N c.
Source:
PAC, 1996, 68, 149. 'A glossary of terms used in chemical kinetics, including reaction dynamics (IUPAC Recommendations 1996)' on page 184 (https://doi.org/10.1351/pac199668010149)