A portion of a macromolecule, comprising many constitutional units that has at least one feature which is not present in the adjacent portions. Where appropriate, definitions relating to 'macromolecule' may also be applied to 'block'.
Source:
PAC, 1996, 68, 2287. 'Glossary of basic terms in polymer science (IUPAC Recommendations 1996)' on page 2298 (https://doi.org/10.1351/pac199668122287)