Nuclear Fission : The process of breaking up of the nucleus of a heavy atom into two, more or less equal segments with the release of a large amount of energy is known as fission.
Nuclear Fission is the splitting up of the nuclear of a heavy atom into two roughly equal fragments, accompanied by the release of energy.
For example — Uranium (U)-235 splits up when is captures a slow neutron according to the fission reaction.
When the atom of U-235 are bombarded by slow motion neutrons, it disintegrates into two parts and release tremendous energy. This process is known as Nuclear Fission.
In this process some new neutrons are generated which collide with atoms of other U-235 and is further disintegrated.
In this way Chain Reaction takes place and enormous amount of energy is released. Atom Bomb is prepared on this principle.