noun

1. Chem. any of two or more chemical compounds having the same constituent elements in the same proportion by weight but differing in physical or chemical properties because of differences in the structure of their molecules
2. Physics any of two or more nuclei possessing the same number of neutrons and protons, but existing in different energy states, and thus having different radioactive properties

Etymology: < Gr isomerēs, equally divided < isos, equal + meros, a part: see merit