An Etymological Dictionary of Astronomy and Astrophysics

Physics: Any of two or more nuclei having the same atomic number A and mass number Z but different half-lives.
Chemistry: One of two or more substances that have the same chemical composition but differ in structural form.

Etymology (EN): Back formation from isomeric, from → iso- + mer a combining form meaning “part,” from Gk. meros “part, portion, share.”

Etymology (PE): Izomer, loan from Fr. isomère.