Illustrated Glossary of Organic Chemistry

Chain termination (termination): A chain reaction mechanism step in which one or more chain-carrying species react without producing another chain-carrying molecule. Most commonly encountered as a free radical reaction mechanism step having radical reactant(s) but no radical product(s). (Note that not all mechanism steps which consume radicals without producing new radicals are termination steps. Some can be propagation steps.)

Reaction Mechanism for Free Radical Bromination of Methane

Photolysis of molecular bromine is a chain initiation step.

Hydrogen atom abstraction by bromine radical is a chain propagation step.

Bromine atom abstraction is a chain propagation step.

Several chain termination events are also occurring.