Real Dictionary

chain reaction
countable noun: A chain reaction is a series of chemical changes, each of which causes the next.
countable noun: A chain reaction is a series of events, each of which causes the next.

chain reaction in American English
a self-sustaining series of chemical or nuclear reactions in which the products of the reaction contribute directly to the process: it can be started by light, an electric spark, bombardment with neutrons, etc.
noun: a self-sustaining reaction in which the fission of nuclei of one generation of nuclei produces particles that cause the fission of at least an equal number of nuclei of the succeeding generation

chain reaction in British English
noun: a process in which a neutron colliding with an atomic nucleus causes fission and the ejection of one or more other neutrons, which induce other nuclei to split

chain reaction in Chemical Engineering
noun: A chain reaction is a chemical or nuclear reaction in which the products of the reaction cause more reactions to happen.