The Collaborative International Dictionary of English v.0.48:

Tetrachord \Tet"ra*chord\, n. [L. tetrachordon, Gr. ?, from ?
   four-stringed; te`tra- (see Tetra-) + ? a chord: cf. F.
   t['e]trachorde.] (Anc. Mus.)
   A scale series of four sounds, of which the extremes, or
   first and last, constituted a fourth. These extremes were
   immutable; the two middle sounds were changeable.
   [1913 Webster]