In synchronous digital electronics, such as most computers, a clock signal is a signal used to coordinate the actions of two or more circuits. A clock signal oscillates between a high and a low state, normally with a 50% duty cycle. In other words, the signal is a square wave. The circuits using the clock signal for synchronization may become active at either the rising or falling edge, or both (see for example DDR SDRAM), of the clock signal.
Most integrated circuits of sufficient complexity require a clock signal in order to synchronize different parts of the chip and to account for propagation delays. As chips get more complex, the problem of supplying accurate and synchronized clocks to all the circuits becomes more and more difficult. The preeminent example of such complex chips is the microprocessor, the central part of modern computers.
The speed of a clock signal in a computer is called the clock rate or clock frequency.