When in a chain reaction there is a net increase in the number of chain carriers there is said to be chain branching. A simple example of a chain-propagating reaction leading to chain branching is:
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in which there is one chain carriers (an oxygen atom) on the left and two chain carriers (a hydrogen atom and a hydroxyl radical) on the right.
See also: degenerate chain branching
Source:
PAC, 1996, 68, 149. 'A glossary of terms used in chemical kinetics, including reaction dynamics (IUPAC Recommendations 1996)' on page 156 (https://doi.org/10.1351/pac199668010149)