核同质异能素（亦称同核异构体）指的是由于某个原子的原子核内核子（质子或中子）处于激发态，而产生原子核的亚稳态，这种状态下原子核内的核子会占用能量更高的核子轨道。这通常是核反应的产物。由于这些在激发态的核子的半衰期比常见的激发态的核子的半衰期要长（通常达到100~1000倍的时间），因此被称作处于“亚稳态”（英语：Metastability），并在原子的质量数后附上“m”作为标记，如58m
27Co。在有多个亚稳态时，使用m1、m2、m3等，按照激发能量从低到高进行标记，如177m2
72Hf。通常，这一术语只指那些半衰期在10秒以上的状态，一些学术文章中更是推荐以5×10秒作为最短的半衰期。

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 seconds). As a result, the term "metastable" is usually restricted to refer to isomers with half-lives of 10 seconds or longer. Some sources recommend 5 × 10 s to distinguish the metastable half life from the normal "prompt" gamma emission half life.