Definition: Nuclear fission is a nuclear reaction in which a heavy nucleus (such as uranium) splits into two lighter nuclei (and possible some other radioactive particles as well).
In such radioactive heavy nuclei, the balance between the strong nuclear force attractive force and the electrostatic repulsive force can be knocked out of equilibrium, by the introduction of energy in the form of an absorbed neutron or photon, the nucleus oscillates in an attempt to regain equilibrium until the electrostatic force gains more power than the shorter-distanced nuclear force, at which point the nucleus splits apart, releasing energy as it does so.
