A chordophone is any musical instrument which produces sound primarily by way of a vibrating string or strings stretched between two points. It is one of the four main divisions of instruments in the original Hornbostel-Sachs scheme of musical instrument classification, the other three being idiophone, membranophone and aerophone (a fifth division, electrophone, was added in a later revision).

What most westerners would call string instruments are classified as chordophones, violins, guitars, lyres, harps, for example. However, the word also embraces instruments that many westerners would hesitate to call string instruments, such as the musical bow and the piano (which, although sometimes called a string instrument, is also called a keyboard instrument and a percussion instrument).

Hornbostel-Sachs divides chordophones into two main groups: instruments without a resonator which is an integral part of the instrument (which have the classification number 31); and instruments with such a resonator (which have the classification number 32). Most western instruments fall into the second group, but the piano and harpsichord fall into the first. Hornbostel and Sachs' criterion for determining which sub-group an instrument falls into is that if the resonator can be removed without destroying the instrument, then it is classified as 31. The idea that the piano's casing, which acts as a resonator, could be removed without destroying the instrument, may seem odd, but if the action and strings of the piano were taken out of its box, it could still be played. This is not true of the violin, because the string passes over a bridge located on the resonator box, so removing the resonator would mean the strings had no tension.