The Collaborative International Dictionary of English v.0.48:

monomer \mon"o*mer\, n. (Chem.)
   The basic conceptual building unit of a polymer; a molecule
   of low molecular weight which may combine with other
   molecules to form a molecule in a chain or branched form
   having high molecular weight; as, amino acids are the monomer
   units which are combined to form proteins; vinylic plastics
   are formed from monomers having a vinyl group.
   [PJC]

WordNet (r) 3.0 (2006):

monomer
    n 1: a simple compound whose molecules can join together to form
         polymers