1. (Science: chemistry) One of two or more molecules that have the same chemical formula but have a different stereochemical arrangement of their atoms.
2. (Science: radiobiology) nuclides having the same number of neutrons and protons but capable of existing, for a measurable time, in different quantum states with different energies and radioactive properties. Commonly, the isomer of higher energy decays to one of lower energy by the process of isomeric transition.