heavy water

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

• noun Either of two isotopic forms of water, especially deuterium oxide, consisting chiefly of molecules that contain deuterium instead of protium. Heavy water is used as a moderator in certain nuclear reactors.

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

• noun chemistry Water containing deuterium instead of normal hydrogen (symbol D2O); used as a moderator in nuclear reactors.

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

• noun water containing a substantial proportion of deuterium atoms, used in nuclear reactors