A sequence of events in which each event causes further similar events to occur. In a nuclear reactor, this refers to the series of nuclear fissions initiated by neutrons, where neutrons released from one fission go on to cause further fissions.

A nuclear reaction in which the nucleus of an atom splits into smaller parts, often producing free neutrons and releasing a large amount of energy. This process is fundamental to the operation of nuclear power plants and nuclear weapons.

The state in a nuclear reactor where each fission event releases enough neutrons to sustain a continuous series of reactions, achieving a self-sustaining chain reaction. This is a crucial milestone in the operation of a nuclear reactor.