In a chain reaction, the average number of times the closed cycle of reactions (involving the chain-propagating reactions) is repeated. It is equal to the rate of the overall reaction divided by the rate of the initiation reaction.
Source:
PAC, 1996, 68, 149. 'A glossary of terms used in chemical kinetics, including reaction dynamics (IUPAC Recommendations 1996)' on page 157 (https://doi.org/10.1351/pac199668010149)