A MIDI message that tells the receiver to stop sounding a note, or more specifically, to transition the note to the release phase of its envelope. (If the release time in the current patch is long, it may be some time before the note actually stops sounding.) The message contains the note number of the note to release, and an extra byte which may contain a release velocity.
Although the situation should not happen with a keyboard, it is possible for some alternative controllers and sequencers to send a note on message with the number of a note that is already sounding and has not yet been released by a note off message (a situation called "double voicing"). In order to allow for the different ways that a polyphonic synth might handle this situation, a design principle is that every note on message must be paired with a note off. This can lead to situations where the synth receives a redundant note off message for a note number. Additionally, an all notes off message may have released voices prior to their note off messages arriving. For this reason, a synth should always discard a note off message that specifies a note that has already been released.