A MIDI message that is sent out by a timing source in order to mark the passage of time to the receiving unit or units. The MIDI Clock message was defined in the original 1980s MIDI standard as a primitive method of achieving synchronization between sequencers and drum machines, tape units, etc.; it predates the MIDI time code. The unit providing the time reference sends out 24 MIDI Clock messages per quarter-note beat. A MIDI Clock message is only a “tick” that marks the passage of time. It does not actually tell the time, which means that it can maintain units in synchronization but cannot automatically put units in sync unless they are synchronized by other means prior to starting up, unlike MIDI time code. The compensating factor is that MIDI clock messages are short and so don't take up much bandwidth, and they are easier for a sync converter to convert to to other forms of synchronization such as DIN sync.
Math example: Assuming a tempo of 130 beats per minute (a common tempo in dance electronica):
MIDI clocks per minute
MIDI clocks per second
milliseconds per MIDI clock