Monochord

MON'OCHORD, n. [Gr. sole, only, and chord.] A musical instrument of one string. As its name imports, it had originally but one string; but it is generally constructed with two, by means of which the musician is better enabled to try the proportions of sounds and intervals, and judge of the harmony of two tempered notes.

In the proper sense of the word, a trumpet marine is considered a monochord.