MIDI (Musical Instrument Digital Interface) - definition
A set of specifications allowing computers, synthesizers, MIDI controllers, sound cards, samplers and drum machines to control one another and exchange system data.
MIDI files keep information that describes the instruments, notes and timing of the music. This can then be recreated on MIDI-capable devices as music.
More sophisticated MIDI devices can not only reproduce consecutive notes (monophony) but are able to create realistic-sounding music by synthesizing several notes simultaneously - polyphony. The more notes the synthesizer can play simultaneously, the nicer it sounds.
MIDI files were commonly used as mobile phone ringtones before the support for the MP3/AAC standard was widely adopted.