The minimum mass of a fissionable material that will just maintain a fission chain reaction under precisely specified conditions, such as the nature of the material and its purity, the nature and thickness of the tamper (or neutron reflector), the density, and the physical shape. For an explosion to occur, the system must be supercritical (i.e., the mass of the material must exceed the critical mass under the existing conditions).

See Fission, Supercritical.