In organic chemistry, isomerism is the existence of two or more chemical compounds with the same molecular formula but having different properties owing to a different arrangement of atoms within the molecule. E.g., ammonium cyanate, NH₄CNO, and urea, CO(NH₂)₂, are isomers. See also tautomerism.

In nuclear physics, nuclei having the same atomic number, but which exist in different energy states are said to be isomeric. E.g., a nucleus in its ground state and a nucleus in a metastable excited state are isomers. Nuclear isomers are formed by bombardment or in a radioactive decay chain. They usually have very short half-lives and decay by emitting gamma rays.