isomer - n.
1 Chem. one of two or more compounds with the same molecular formula but a different arrangement of atoms and different properties.
2 Physics one of two or more atomic nuclei that have the same atomic number and the same mass number but different energy states.

isomeric adj. isomerism n. isomerize v. (also -ise). [G f. Gk isomeres sharing equally (as ISO-, meros share)]