Technically, the process of sync generator locking. The term is commonly used in the video discipline to the synchronization of video signals from one device with those of another video source. This is required when mixing signals together, as in overlaying multiple images or computer graphics on an image from a camera, VCR or videodisc player to prevent screen flicker or rolling.

Genlock is usually performed by introducing a composite video signal from a synchronizer – a master source (see WFTD Black Burst) – to the subject, or slave, sync generator. Then the slave is set to lock up to, or follow, the master so that both sync generators are running at the same frequency and phase.