The process in which the heavy nucleus of a radioactive atom(such as uranium) splits up into smaller nuclei when bombarded with low energy neutrons, is called nuclear fission.

E.g., When uranium-235 atoms are bombarded with slow moving neutrons, the heavy uranium nucleus breaks up to produce two medium-weighted atoms and 3 neutrons, with the emission of tremendous amount of energy.

\(_{92}^{235}U + _{0}^{1}n \rightarrow + Fission \) \(_{56}^{13}Ba + _{36}^{94}Kr + 3_{0}^{1}n \) + tremendous amount of energy