WordNet (r) 3.0 (2006):

MHz
    n 1: one million periods per second [syn: megahertz, MHz,
         megacycle per second, megacycle, Mc]

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

MegaHertz
MHz

   (MHz) Millions of cycles per second.  The unit of frequency
   used to measure the clock rate of modern digital logic,
   including microprocessors.

   (1994-10-27)