A macromolecule in which a small but significant proportion of the constitutional units have ionizable or ionic groups, or both. Some protein molecules may be classified as ionomer molecules.
Source:
PAC, 1996, 68, 2287. 'Glossary of basic terms in polymer science (IUPAC Recommendations 1996)' on page 2299 (https://doi.org/10.1351/pac199668122287)