SMPTE timecode
Hours, minutes, seconds and frames — the address that gives every frame in a production a unique name.
The Society of Motion Picture and Television Engineers standardised the HH:MM:SS:FF format around 1970 so that videotape editing could be controlled precisely rather than by a finger on the reel. The code is recorded as an audio-like signal on its own track (LTC) or in the picture's vertical blanking interval (VITC), and in the music studio it travels over MIDI as MIDI Time Code, letting a DAW follow a film frame by frame.
The notorious detail is drop-frame. American colour television runs at 29.97 frames per second, not 30, so a counter assuming 30 drifts by 3.6 seconds an hour. The fix is to skip frame numbers 00 and 01 every minute except every tenth. No frames are lost, only numbers — a fact explained to every new assistant editor, every year.