What is the meaning of Monochord?

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.

A stringed instrument with only one string.

A bass note on a piano that has a single string.

Source: wiktionary.org