Monomer
A monomer (/ˈmɒnəmər/ MON-ə-mər; mono-, "one" + -mer, "part") is a molecule that can react together with other monomer molecules to form a larger polymer chain or two- or three-dimensional network in a process called polymerization.
IUPAC definition
Monomer molecule: A molecule which can undergo polymerization, thereby contributing constitutional units to the essential structure of a macromolecule.