A clock cycle, or simply a "cycle," is a single electronic pulse of a CPU. During each cycle, a CPU can perform a basic operation such as fetching an instruction, accessing memory, or writing data. Since only simple commands can be performed during each cycle, most CPU processes require multiple clock cycles.
In physics, the frequency of a signal is determined by cycles per second, or "hertz." Similarly, the frequency of a processor is measured in clock cycles per second. Since modern processors can complete millions of clock cycles every second, processor speeds are often measured in megahertz or gigahertz.
The frequency of a processor is also known as the processor's clock speed. While the clock speed is important in determining the processor's overall performance, it is not the only factor. Since processors have different instruction sets, they may differ in the number of cycles needed to complete each instruction, or CPI (cycles per instruction). Therefore, some processors can perform faster than others, even at slower clock speeds.