Source : Free On-Line Dictionary of Computing

clock rate
     
         The fundamental rate in {cycle}s 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)