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

overclocking

    Any adjustments made to computer hardware, or less
   commonly software, to make its CPU run at a higher clock
   rate than intended by the original manufacturers.  Typically this
   involves replacing the crystal in the clock generation
   circuitry with a higher frequency one or changing jumper
   settings or software configuration.

   If the clock rate is increased too far, eventually some
   component in the system will not be able to cope and the system
   will stop working.  This failure may be intermittent (it works
   most of the time but fails more often than usual) continuous (the
   system never works at the higher frequency) or, in the worst case,
   irreversible (a component is damaged by overheating).  Improved
   cooling may be needed to maintain the same level of reliability.

   (2014-07-12)