Any musical instrument that makes sound by way of a vibrating string or strings stretched between two points. Hence, all string instruments are chordophones. However, the word also embraces instruments that are not obviously string instruments, such as the mouth bow and the piano (which, although sometimes called a string instrument, is more usually referred to as a keyboard instrument).

Chordophones may be subdivided into those with a resonator and those without. Most Western instruments fall into the first of these, but the piano and harpsichord are in the second.

Electric string instruments usually have an electromagnetic pickup that produces a signal that can be amplified.