From 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]