Processes in which a heavier atomic nucleus splits up into several lighter nuclei. If the initial nucleus is heavy enough, energy is set free in the split. Nuclear fission reactions are used in nuclear reactors to produce electrical energy, and in nuclear weapons to power an energetic explosion.
Some more information about nuclear fission can be found in the spotlight topic Is the whole the sum of its parts?. The role played in nuclear fission and fusion by Einstein’s famous formula E=mc² is the subject of the spotlight topic From E=mc² to the atomic bomb.