WordNet (r) 3.0 (2006):

heat sink
    n 1: a metal conductor specially designed to conduct (and
         radiate) heat

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

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)