Nuclear fission is a nuclear reaction in which the nucleus of an atom splits into smaller parts (lighter nuclei).The fission process often produces free neutrons and photons (in the form of gamma rays), and releases a large amount of energy.

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.