monochord
noun
mono·chord ˈmä-nə-ˌkȯrd 
: an instrument of ancient origin for measuring and demonstrating the mathematical relations of musical tones and that consists of a single string stretched over a sound box and a movable bridge set on a graduated scale
Example Sentences
Recent Examples on the Web With gongs, crystal bowls, Tibetan singing bowls, Native American flute, didgeridoo, Celtic Harp, monochord, vibraphone. 
San Francisco Chronicle, 24 May 2018
San Francisco Chronicle, 24 May 2018 Word History
Etymology
Middle English monocorde, from Middle French, from Medieval Latin monochordum, from Greek monochordon, from mon- + chordē string — more at yarn
First Known Use
15th century, in the meaning defined above
Medical Definition
monochord
noun
mono·chord ˈmän-ə-ˌkȯ(ə)rd
: an instrument that has been used to test hearing acuity and that consists of a single string stretched over a sounding board and a movable bridge set on a graduated scale