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
