Audio & Video: Signal to start or stop an operation or to move to a predetermined point in programme material.
On tape this will involve spooling and, maybe, a reel change. On disk it can be instant.
Film: Change-over-cues: Marks, at the end of a reel of film intended for projection, to indicate when to start the motor of the incoming projector and when to switch over picture and sound.
Pank, B., Editor, 1996, The Digital Fact Book, 8th Ed, Quantel, Newbury, UK