A situation in which one action causes or initiates a similar action. In a nuclear chain reaction, for example, a neutron strikes a uranium-235 nucleus, causing the nucleus to undergo fission, which in turn produces a variety of products. Among these products is one or more neutrons. Thus, the particle needed to initiate this reaction (the neutron) is itself produced as a result of the reaction. Once begun, the reaction continues as long as uranium-235 nuclei are available. Nuclear chain reactions are important sources of fission and fusion energy.

A chain reaction. (Illustration by Laurence Lawson. (Reproduced by permission.)