The Collaborative International Dictionary of English v.0.48:

Octachord \Oc"ta*chord\, n. [Gr. ? with eight strings; 'okta-
   (for 'oktw` eight) + ? string, chord: cf. F. octacorde.]
   (Mus.)
   An instrument of eight strings; a system of eight tones.
   [Also written octochord.]
   [1913 Webster]