MIDI is not a thing, a device or an item. MIDI is a "standard". A group of specifications or a language, if you will, which allows electronic devices like keyboards, computers, processors, software, and other equipment to operate together.
MIDI stands for Musical Instrument Digital Interface. It was developed in the eighties, and finalized in 1984. MIDI allows products from different manufacturers to "communicate" with each other using a standardized system. Some of the things that the MIDI specification covers are: Note on, Note off, Note pitch (which is represented as a number from 0 to 127), Key Velocity, After Touch, Pitch Bend, and Program Change to name a few.
As the MIDI specification developed it also included things like:
- Time Code - Allows electronic musical instruments to stay synchronized with studio audio equipment.
- Show Control - Allows lights and other stage equipment to work together.
- Machine Control - Allows tape decks and video recording and playback equipment to be controlled and work with electronic musical instruments.