A nuclear isomer is a metastable state of an atomic nucleus in which one or more nucleons occupy excited state levels. "Metastable" describes nuclei whose excited states have half-lives of 10−9 seconds or longer, 100 to 1000 times longer than the half-lives of the excited nuclear states that decay with a "prompt" half-life. Some references recommend using a threshold of 5×10−9 seconds to distinguish the metastable half-life from the normal "prompt" gamma-emission half-life.