Illustrated Glossary of Organic Chemistry

Chain propagation (propagation): A free radical reaction mechanism step having radical reactant(s) and gives radical product(s).

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: