A @C01284@ in which the @R05260@ or @A00020@ is defined at one or more sites of @S05983@ in each @C01288@ in the main chain of a @P04735@ molecule.
Source: Purple Book, 1st ed., p. 36 [Terms] [Book]
Cite as: IUPAC. Compendium of Chemical Terminology, 2nd ed. (the "Gold Book"). Compiled by A. D. McNaught and A. Wilkinson. Blackwell Scientific Publications, Oxford (1997). Online version (2019-) created by S. J. Chalk. ISBN 0-9678550-9-8. https://doi.org/10.1351/goldbook.