heavy water

water with a higher average molecular weight than ordinary water, water used for cooling the core of an atomic reactor

Heavy water is water which contains a higher proportion than normal of the isotope deuterium, as deuterium oxide, D2O or 2H2O, or as deuterium protium oxide, HDO or 1H2HO. Its physical and chemical properties are somewhat similar to those of water, H2O. Heavy water may contain as much as 100 2O, and usually the term refers to water which is highly enriched in deuterium. The isotopic substitution with deuterium alters the bond energy of the hydrogen-oxygen bond in water, altering the physical, chemical, and especially biological properties of the pure or highly-enriched substance to a larger degree than is found in most isotope-substituted chemical compounds.
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Noun
1. water containing a substantial proportion of deuterium atoms, used in nuclear reactors
(synonym) deuterium oxide
(hypernym) moderator

Water containing significantly more than the natural proportions (one in 6,500) of heavy hydrogen (deuterium, D) atoms to ordinary hydrogen atoms. Heavy water is used as a moderator in some reactors because it slows down neutrons effectively and also has a low probability of absorption of neutrons.

(D2O) Water that contains 2H, rather than 1H. Heavy water is about 11% denser than ordinary water.