1. Physics.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.
2. Chem.a reaction that results in a product necessary for the continuance of the reaction.
3. a series of events in which each event is the result of the one preceding and the cause of the one following.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.