What is the process by which a nucleus splits into two smaller nuclei?

The process by which a nucleus splits into two smaller nuclei is called nuclear fission.

In fission, a heavy atomic nucleus divides into two major fragments, usually releasing several neutrons and a large amount of energy. The products are different elements from the original atom, so fission is also a form of nuclear transmutation. Much of the released energy appears as kinetic energy of the fragments, which becomes heat.

Fission may happen spontaneously, but it is often induced when a neutron strikes a heavy nucleus such as uranium-235. The resulting neutrons can trigger additional fissions, creating a chain reaction. Nuclear reactors control such reactions; nuclear weapons use an extremely rapid, uncontrolled chain reaction.

Fission is not the same as radioactive decay in general: radioactive decay is the broader category, while fission is one possible nuclear process. It is also the opposite-style process from nuclear fusion, in which light nuclei combine. The discovery was established in 1938 by Otto Hahn and Fritz Strassmann, while Lise Meitner and Otto Frisch supplied the theoretical explanation in 1939.