monochord

noun3

1. •
   A musical instrument for experimenting with the mathematical relations of musical sounds, consisting of a single string stretched between two bridges, one or both of which can be moved, and which stand upon a graduated rule for the purpose of changing and measuring the length of the part of the string between them.

   • If a musician were going to give a lecture upon the mathematical part of his art, he would find a very elegant substitute for the monochord in the Chinese kin. — 1840
2. •
   A stringed instrument with only one string.
3. •
   A bass note on a piano that has a single string.