Source : Free On-Line Dictionary of Computing

overclocking
     
         Any adjustments made to computer hardware (or
        software) to make its CPU run at a higher {clock} frequency
        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 frequency 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 continuous (the
        system never works at the higher frequency) or intermittant
        (it fails more often but works some of the time) or, in the
        worst case, irreversible (a component is damaged by
        overheating).  Overclocking may necessitate improved cooling
        to maintain the same level of reliability.
     
        (1999-09-12)