Musical Instrument Digital Interface
(MIDI /mi'-dee/, /mee'-dee/) A hardware specification and protocol used to communicate note and effect information between synthesisers, computers, music keyboards, controllers, and other electronic music devices. It is basically a high-speed serial connection with separate connections for MIDI in, MIDI out and MIDI through (to allow devices to be chained).
The basic unit of information is a "note on/off" event which includes a note number (pitch) and key velocity (loudness). There are many other message types for events such as pitch bend, patch changes and synthesizer-specific events for loading new patches etc.