Definition
Musical Instrument Digital Interface (MIDI) is a protocol for electronic instruments, computers and other devices to exchange performance data such as notes, velocity and control messages. Released in 1983 and maintained by the MIDI Association, MIDI carries instructions rather than audio itself.
How it works
When a key is pressed, the device sends a MIDI message—“this channel, this pitch, this velocity, pressed”—and the receiving instrument sounds it with its own voice. The same stream of messages can make completely different instruments play the identical performance.
Usage
MIDI is the backbone of electronic music production: connecting keyboards, synthesizers and DAWs, sequencing parts, and linking gear in live performance all run on MIDI messages or MIDI files.
Related and confusable terms
MIDI records how something is played, while audio records the sound itself—which is why MIDI files are tiny and their instruments can be changed freely. BPM, the measure of tempo, is a common parameter when working with MIDI sequences.