An industry internationally agreed standard for the serial interconnection of control signals between electronic musical instruments. It is mainly used for the transfer of musical performance data, such as which notes are played and how rapidly they are depressed and released (coded as note-on and note-off messages that incorporate note-velocity information), and the state of performance controllers, such as volume controls, pitch bend, and modulation wheels. MIDI data can be sent to and from computer sequencer systems for storage and modification. MIDI also enables instrument-specific, or ‘system-exclusive’, data to be communicated relating to internal instrument settings and digital audio samples.

The MIDI standard defines a unidirectional asynchronous serial interface at 31.25 kilobaud, opto-isolated at the MIDI input of every MIDI device. All devices equipped with MIDI will have a MIDI input (IN), some will have a MIDI output (OUT) if MIDI data is generated by the device, and many provide a buffered version of the MIDI input at a MIDI THRU socket for connection to other MIDI devices in the chain.