The Collaborative International Dictionary of English v.0.48:

octapeptide \oc`ta*pep"tide\ ([o^]k`t[u^]*p[e^]p"t[imac]d), n.
   [Octa- + peptide.] (Chem.)
   A molecule composed of eight amino acid units bound to each
   other by peptide bonds, usually in a linear array. See
   octamer.
   [PJC]