MIDI clock
A stream of 24 pulses per quarter note that keeps drum machines and sequencers at the same tempo — without ever knowing what time it is.
The master sends the byte F8 24 times per quarter note; the tempo is simply how closely the pulses are spaced. FA, FB and FC mean start, continue and stop, and the Song Position Pointer says where in the song you are. Everything is counted in musical beats, not seconds, which suits a drum machine perfectly and a film editor not at all — that's what MIDI Time Code is for, carrying SMPTE timecode over the same cable.
The weak spot is jitter. One pulse every 20 milliseconds at 125 BPM is coarse resolution, and a software DAW sending clock through a USB interface can wobble audibly. Plenty of hardware musicians therefore let a dedicated box be master.