Also called clock rate, the speed at which a microprocessor executes instructions. Every computer contains an internal clock that regulates the rate at which instructions are executed and synchronizes all the various computer components. The CPU requires a fixed number of clock ticks (or clock cycles) to execute each instruction. The faster the clock, the more instructions the CPU can execute per second.
The internal architecture of a CPU has as much to do with a CPU's performance as the clock speed, so two CPUs with the same clock speed will not necessarily perform equally. Whereas an Intel 80286 microprocessor requires 20 cycles to multiply two numbers, an Intel 80486 or later processor can perform the same calculation in a single clock tick. (Note that clock tick here refers to the system's clock, which runs at 66 MHz for all PCs.) These newer processors, therefore, would be 20 times faster than the older processors even if their clock speeds were the same. In addition, some microprocessors are superscalar, which means that they can execute more than one instruction per clock cycle.
Like CPUs, expansion buses also have clock speeds. Ideally, the CPU clock speed and the bus clock speed should be the same so that neither component slows down the other. In practice, the bus clock speed is often slower than the CPU clock speed, which creates a bottleneck. This is why new local buses, such as AGP, have been developed.