ETYM Latin fissio. Related to Fissure.
1. A nuclear reaction in which a massive nucleus splits into smaller nuclei with the simultaneous release of energy; SYN. nuclear fission.
2. Reproduction of some unicellular organisms by division of the cell into two more or less equal parts.
3. The splitting of the nucleus of an element into fragments. Heavy elements such as uranium or plutonium release energy when fissioned.
Physics, splitting, especially of atomic nuclei, accompanied by release of immense energies. In physics, the splitting of a heavy atomic nucleus into two or more major fragments. It is accompanied by the emission of two or three neutrons and the release of large amounts of nuclear energy.
Fission occurs spontaneously in nuclei of uranium-235, the main fuel used in nuclear reactors. However, the process can also be induced by bombarding nuclei with neutrons because a nucleus that has absorbed a neutron becomes unstable and soon splits. The neutrons released spontaneously by the fission of uranium nuclei may therefore be used in turn to induce further fissions, setting up a chain reaction that must be controlled if it is not to result in a nuclear explosion.

1. To undergo fission
2. To cause to undergo fission