A device in which a self-sustaining nuclear fission chain reaction can be maintained and controlled. The term is sometimes applied to a device in which a nuclear fusion reaction can be produced and controlled.
Source:
PAC, 1982, 54, 1533. 'Glossary of terms used in nuclear analytical chemistry (Provisional)' on page 1550 (https://doi.org/10.1351/pac198254081533)