The Free On-line Dictionary of Computing (30 December 2018):

clock rate
clock frequency
clock speed

    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 486 CPU 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.

   (1995-01-12)