An Etymological Dictionary of Astronomy and Astrophysics

A → nuclear reaction in which a heavy atomic nucleus splits into two or more approximately equal parts, usually as the result of the capture of a slow, or → thermal neutron
by the nucleus. It is normally accompanied by the emission of further neutrons or → gamma rays and very large amounts of energy. The neutrons can continue the process as a → chain reaction, so that it becomes the source of energy in a → nuclear reactor or an atomic bomb. It may also be a trigger for → nuclear fusion in a hydrogen bomb. Fission occurs spontaneously in nuclei of uranium-235, the main fuel used in nuclear reactors.

See also: → nuclear; → fission.