heat shield

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

• noun A barrier that prevents the heating of a space or an object by absorbing, reflecting, or dissipating external heat, especially a protective structure on a spacecraft or missile that dissipates heat on atmospheric reentry by melting and vaporizing.

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

• noun A shield designed of a substance that absorbs excessive heat from an outside source by either dissipating, reflecting or simply absorbing the heat.

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

• noun a protective covering that protects a spacecraft from overheating on reentry