chain reaction, 1. (in chemistry) a reaction that proceeds through one or more reactive intermediates; one of the required reactive intermediates (usually free radicals) is formed in each step of the reaction. Examples include the polymerization of organic monomers into plastics or in the free radical halogenation of hydrocarbons. 2. (in physics) a reaction that perpetuates itself by the proliferating fission of nuclei and the release of atomic particles that cause more nuclear fissions.
