A timing source. In digital audio or video a clock serves as a timing reference for a system. Everything in digital audio and video is related to a clock. There are specified numbers of operations per period of time and they all must be carried out at a consistent speed in order for the device to work properly. In digital audio the relevant clock is the sample rate, or word clock of the audio (44.1 kHz, 48kHz, etc). In video it may be the frame rate of the video, or the field rate, or some other video specific rate. The clock or timing of information is an ongoing issue when synchronizing systems with each other.