One clock cycle, or simply a "cycle" is an electronic pulse of a single processor. During each cycle, a processor can perform a basic operation such as retrieving an instruction, access to memory or writing data. Since simple commands can be performed at each cycle, the majority of CPU processes require several clock cycles.
In physics, the frequency of a signal is determined by the cycles per second, or "Hertz". Similarly, the frequency of a processor is measured in clock cycles per second. Since modern processors may complete millions of clock cycles each second processor speeds, are often measured in megahertz or gigahertz.
The frequency of a processor is also known that the clock speed of the processor. Although the clock speed is important in determining the overall performance of the processor, it is not the only factor. Since processors have instruction sets, they may differ in the number of cycles required to complete each instruction, or CPI (cycles per instruction). Therefore, some processors can operate more quickly than others, even at slower clock speeds.