0 any musical instrument in which sound is produced mainly by means of electricity: --
Keyboards are integral to many aerophones (eg the accordion), chordophones (eg the piano), and electrophones (eg the synthesizer).
It can be debated whether the pipe organ (which uses electric key action to control solenoid valves) is an electrophone or an aerophone.
A fifth class of musical instrument has been added: electrophones, in which the sound is produced by electromagnets, oscillators, or other non-acoustic devices.
In an electrophone, the sound may be generated by mechanical or electronic oscillators (eg a synthesizer), or by acoustically generated vibrations that require electrical amplification (eg the electric guitar).
The electrophone was not in the original Hornbostel-Sachs scheme of musical instruments published in 1914.