Source : WordNet®

heat sink
     n : a metal conductor specially designed to conduct (and
         radiate) heat

Source : Free On-Line Dictionary of Computing

heat sink
     
         /heet sink/ (from "sink", electronics jargon for
        something which takes in current) A piece of thermally
        conductive metal attached to a {semiconductor} or other
        electronic device and designed to prevent it from overheating
        by conducting heat away from it and radiating it to the
        environment.  Heat sinks often have fins to increase their
        surface area.  They occasionally have fans attached.  Heat
        sink compound can be smeared between the device and the heat
        sink to improve thermal conduction.
     
        (1997-06-10)