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 excited nuclear states that decay with a "prompt" half life (ordinarily 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. Some sources recommend 5 × 10^-9 s to distinguish the metastable half life from the normal "prompt" gamma emission half life.