The Free On-line Dictionary of Computing (30 December 2018):
The fundamental rate in cycles per
second at which a computer performs its most basic operations
such as adding two numbers or transfering a value from one
register to another.
The clock rate of a computer is normally determined by the
frequency of a crystal. The original IBM PC, circa 1981,
had a clock rate of 4.77 MHz (almost five million
cycles/second). As of 1995, Intel's Pentium chip runs at
100 MHz (100 million cycles/second). The clock rate of a
computer is only useful for providing comparisons between
computer chips in the same processor family. An IBM PC
with an Intel 486CPU running at 50 MHz will be about
twice as fast as one with the same CPU, memory and display
running at 25 MHz. However, there are many other factors to
consider when comparing different computers. Clock rate
should not be used when comparing different computers or
different processor families. Rather, some benchmark should
be used. Clock rate can be very misleading, since the amount
of work different computer chips can do in one cycle varies.
For example, RISC CPUs tend to have simpler instructions
than CISC CPUs (but higher clock rates) and pipelined
processors execute more than one instruction per cycle.