Any adjustments made to computer hardware (or 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 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.