heat sink

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun An environment capable of absorbing heat from an object with which it is in thermal contact without a phase change or an appreciable change in temperature.
• noun A protective device that absorbs and dissipates the excess heat generated by a system.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun thermodynamics An object, system or environment that absorbs and dissipates heat from objects in thermal contact (either direct or radiant) with it.
• noun thermodynamics A place toward which the heat moves in a system.
• noun thermodynamics A material capable of absorbing heat.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a metal conductor specially designed to conduct (and radiate) heat