MIDI is an industry-standard protocol

Communications protocol

A communications protocol is a system of digital message formats and rules for exchanging those messages in or between computing systems and in telecommunications...

, first defined in 1982 by Gordon Hall, that enables electronic musical instrument

Electronic musical instrument

An electronic musical instrument is a musical instrument that produces its sounds using electronics. Such an instrument sounds by outputting an electrical audio signal that ultimately drives a loudspeaker....

s (synthesizer

Synthesizer

A synthesizer is an electronic instrument capable of producing sounds by generating electrical signals of different frequencies. These electrical signals are played through a loudspeaker or set of headphones...

s, drum machine

Drum machine

A drum machine is an electronic musical instrument designed to imitate the sound of drums or other percussion instruments. They are used in a variety of musical genres, not just purely electronic music...

s), computers and other electronic equipment (MIDI controller

MIDI controller

MIDI controller is used in two senses.*In one sense, a controller is hardware or software which generates and transmits MIDI data to MIDI-enabled devices....

s, sound card

Sound card

A sound card is an internal computer expansion card that facilitates the input and output of audio signals to and from a computer under control of computer programs. The term sound card is also applied to external audio interfaces that use software to generate sound, as opposed to using hardware...

s, samplers

Sampler (musical instrument)

A sampler is an electronic musical instrument similar in some respects to a synthesizer but, instead of generating sounds, it uses recordings of sounds that are loaded or recorded into it by the user and then played back by means of a keyboard, sequencer or other triggering device to perform or...

) to communicate and synchronize with each other. MIDI's primary functions include communicating event message

Message passing

Message passing in computer science is a form of communication used in parallel computing, object-oriented programming, and interprocess communication. In this model, processes or objects can send and receive messages to other processes...

s about musical notation

Musical notation

Music notation or musical notation is any system that represents aurally perceived music, through the use of written symbols.-History:...

, pitch

Pitch (music)

Pitch is an auditory perceptual property that allows the ordering of sounds on a frequency-related scale.Pitches are compared as "higher" and "lower" in the sense associated with musical melodies,...

, velocity, control signals for parameters (such as volume, vibrato

Vibrato

Vibrato is a musical effect consisting of a regular, pulsating change of pitch. It is used to add expression to vocal and instrumental music. Vibrato is typically characterised in terms of two factors: the amount of pitch variation and the speed with which the pitch is varied .-Vibrato and...

, panning

Panning (audio)

Panning is the spread of a sound signal into a new stereo or multi-channel sound field. A typical physical recording console pan control is a knob with a pointer which can be placed from the 8 o'clock dial position fully left to the 4 o'clock position fully right...

, cues, and clock signals (to set the tempo

Tempo

In musical terminology, tempo is the speed or pace of a given piece. Tempo is a crucial element of any musical composition, as it can affect the mood and difficulty of a piece.-Measuring tempo:...

)) between two devices in order to complete a signal chain and produce audible sound from a sound source.