Polymer or oligomer the molecules of which are capable of entering, through reactive groups, into further polymerization and thereby contributing more than one structural unit to at least one type of chain of the final polymer.
Source:
PAC, 2004, 76, 889. 'Definitions of terms relating to reactions of polymers and to functional polymeric materials (IUPAC Recommendations 2003)' on page 897 (https://doi.org/10.1351/pac200476040889)