A nuclear isomer is a metastable state of an atomic nucleus caused by the excitation of one or more of its nucleons (protons or neutrons). "Metastable" refers to the fact that these excited states have half-lives more than 100 to 1000 times the half-lives of the other possible excited nuclear states (which ordinarily last on the order of 10⁻¹² seconds). As a result, the term "metastable" is usually restricted to refer to isomers with half-lives of 10⁻⁹ seconds or longer.

Occasionally the half-lives are far longer than this, and can last minutes, hours, or (in one known case 180m
73Ta) at least 10¹⁵ years. Sometimes, the gamma decay from a metastable state is given the special name of an isomeric transition, but save for the long-lived nature of the meta-stable parent nuclear isomer, this process resembles shorter-lived gamma decays in all other aspects.

The first nuclear isomers (Uranium Z/Uranium X₂, now known as 234
91Pa/234m
91Pa) were discovered by Otto Hahn in 1921.

The nucleus of a nuclear isomer occupies a higher energy state than the corresponding non-excited nucleus, which exists in the lowest energy state, called the ground state. In an excited state, one or more of the protons or neutrons in a nucleus occupy a nuclear orbital of higher energy than an available nuclear orbital of lower energy. These states are analogous to excited states of electrons in atoms.