The Jargon File (version 4.4.7, 29 Dec 2003):

overclock
 /oh'vr?klok?/, vt.

    To operate a CPU or other digital logic device at a rate higher than it was
    designed for, under the assumption that the manufacturer put some slop
    into the specification to account for manufacturing tolerances.
    Overclocking something can result in intermittent crashes, and can even
    burn things out, since power dissipation is directly proportional to clock
     frequency. People who make a hobby of this are sometimes called ?
    overclockers?; they are thrilled that they can run their CPU a few percent
    faster, even though they can only tell the difference by running a 
    benchmark program. See also case mod.