A constitutional repeating unit in a regular macromolecule, a regular oligomer molecule, regular block or a regular chain, the configuration of which is defined at least at one site of stereoisomerism in the main chain (backbone).
Source:
PAC, 1996, 68, 2287. 'Glossary of basic terms in polymer science (IUPAC Recommendations 1996)' on page 2291 (https://doi.org/10.1351/pac199668122287)