Definitions for "Heavy water"
Water containing significantly more than the natural proportion (1 in 6500) of heavy hydrogen (deuterium) atoms to ordinary hydrogen atoms. (Hydrogen atoms have one proton, deuterium atoms have one proton and one neutron.) Heavy water is used as a moderator in some reactors because it slows down neutrons effectively and does not absorb them (unlike light, or normal, water) making it possible to fission natural uranium and sustain a chain reaction.
Water in which the hydrogen atoms are the heavy hydrogen isotope, deuterium. It is sometimes called deuterium oxide (D2O).
Water containing a significantly greater proportion of heavy hydrogen (deuterium) atoms to ordinary hydrogen atoms than is found in ordinary (light) water. Heavy water is used as a moderator in some reactors because it slows neutrons effectively and also has a low cross-section for absorption of neutrons.