What is the term for a nuclear reaction where an atom's nucleus is split, releasing large amounts of energy?

The correct term for a nuclear reaction in which an atom's nucleus is split, resulting in a release of large amounts of energy, is nuclear fission. In nuclear fission, a heavy nucleus, such as that of uranium-235 or plutonium-239, absorbs a neutron and becomes unstable, leading to its splitting into smaller nuclei along with the release of energy and additional neutrons. This process is harnessed in nuclear power plants to generate electricity and is also the principle behind atomic bombs.

In contrast, nuclear fusion refers to the process where two light atomic nuclei combine to form a heavier nucleus, a reaction that powers stars, including the Sun, and releases energy as well. Radioactive decay is a broader term that encompasses various processes by which unstable atomic nuclei lose energy by emitting radiation, but it is not specifically about the splitting of nuclei. Alpha decay is a specific type of radioactive decay in which an atomic nucleus emits an alpha particle (helium nucleus) and transforms into a different element; this is also not related to the splitting of the nucleus in the same way as fission.