Clock

Clock (noun) is a signal that is used to trigger or coordinate digital communications or operation. In many types of interface there is a discreet line or pin named "clock" which explicitly performs this function. The active part of the signal is typically the leading edge of the transition, but may be the trailing edge or level in some cases. In other types of signal protocols, the clock is intrinsic or built-in to the signal stream and can be extracted or derived using a simple algorithm. The term "self-clocking" is sometimes used.

The clock signal is used in processors, controllers, input/output circuits, etc. to synchronize operations, and control the speed of processing. Some circuits can operate properly at clock speeds down to zero, while others cannot. Tweaking circuits and environmental factors such as cooling to increase clock speeds beyond the manufacturer's specifications is a popular geek sport, but some ICs have built-in mechanisms to limit or prevent "over-clocking".

Clock (verb) is the action of triggering or signaling the circuit. For example it could be the signal that informs the receiver that the data is valid to be read and held (latched). Or the clock could be a trigger to tell the circuit to perform some function.

Clock Pin is an input, typically to an integrated circuit IC that carries the Clock Signal.