The Collaborative International Dictionary of English v.0.48:

oligomer \o*lig"o*mer\, n. (Chem.)
   A molecule composed of a small number of linked monomer
   units; a short polymer; -- compounds called oligomers have
   less than one hundred monomer units and usually less than
   thirty. Oligomers of increasing length are called dimer,
   trimer, tetramer, pentamer, hexamer, heptamer,
   octamer, nonamer, decamer, etc. In colloquial
   laboratory jargon, they may also be referred to as
   nine-mer, ten-mer, eleven-mer, twelve-mer, etc.,
   especially for oligomers of greater than eight units.
   [PJC]

WordNet (r) 3.0 (2006):

dimer
    n 1: a compound whose molecules are composed of two identical
         monomers