chain reaction science definition

A process in which the result of one event triggers another event, usually of the same kind, which in turn triggers yet another event, so that the overall reaction tends to be self-sustaining. Nuclear fission reactions are chain reactions, in which the splitting of an atomic nucleus releases neutrons that penetrate other nuclei, causing them to split. The spread of heat through a substance is also a chain reaction, as fast-moving molecules in a hot part of the substance collide with neighboring molecules, passing on their kinetic energy to them, thereby making more of the substance warmer. See more at fission. See Note at nuclear reactor. See also kinetic theory.