Source : Free On-Line Dictionary of Computing

word size
     
         The number of bits that a {CPU} can process at one
        time.  Processors with many different word sizes have existed
        though powers of two (8, 16, 32, 64) have predominated for
        many years.  A processor's word size is often equal to the
        width of its external data bus though sometimes the bus is
        made narrower than the CPU (often half as many bits) to
        economise on packaging and circuit board costs.
     
        (1995-04-23)