An Acronym for Musical Instrument Digital Interface.
An eletrical specification for a 30kbps serial line along with a specification for a Protocol for messages such a “play a middle C” and “switch sample to slap guitar.” Widely used to connect musical devices such as keyboards and synthesizers.
It was first proposed in 1981, specified in 1983, and ratified as a standard in 1991. It is very outdated now – positively paleolithic, in fact, esp. the miniscule transfer rate of 30kbps, but also the low number of bits used for many protocol message payloads (eg. you can only select one of 127 samples, so later versions of the spec had to add bank switching between 127 banks to overcome this). Since the industry has never managed to coalesce around any one successor, MIDI remains the only truly interopable standard for connecting digital musical instruments.