What is the process called when a large nucleus splits into two smaller nuclei, releasing energy?

The process in which a large nucleus splits into two smaller nuclei while releasing energy is known as nuclear fission. This reaction typically occurs with heavy elements, such as uranium or plutonium. During fission, the nucleus captures a neutron, which destabilizes it, leading to its splitting into smaller nuclei, accompanied by the release of additional neutrons and a significant amount of energy. This energy release is a result of the conversion of mass into energy, as described by Einstein's equation E=mc².

This process contrasts with nuclear fusion, where two light nuclei combine to form a heavier nucleus and also release energy. Radioactive decay involves the transformation of an unstable nucleus into a different element or a more stable state, while photon absorption refers to the process by which matter absorbs photons, leading to various energy levels but does not involve nuclear reactions. Thus, nuclear fission specifically describes the splitting of a large nucleus and is the correct answer.