A chain-propagating reaction, or more simply a propagating reaction, is an elementary step in a chain reaction in which one chain carrier is converted into another. The conversion can be a unimolecular reaction or a bimolecular reaction with a reactant molecule.
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)