The Collaborative International Dictionary of English v.0.48:

Monochord \Mon"o*chord\, n. [L. monochordon, Gr. ?, fr. ? with
   but one string; ? only, single + ? string: cf. F. monocorde.
   See Chord, and cf. Mainchord.] (Mus.)
   An instrument for experimenting upon the mathematical
   relations of musical sounds. It consists of a single string
   stretched between two bridges, one or both of which are
   movable, and which stand upon a graduated rule for the
   purpose of readily changing and measuring the length of the
   part of the string between them.
   [1913 Webster]