Heavy water is water (H2O) in which oxygen is bound to atoms of the hydrogen isotope deuterium (2H). Heavy water is so named because it is significantly more dense (>1.1 g/cm3) than ordinary ("light") water, 1H2O (1 gm/cm3).

Heavy water is not radioactive and has the same chemical properties as light water; a person could drink a glass of heavy water without harm. However, heavy water is better than light water at moderating (slowing) neutrons, which makes it useful in some nuclear reactor cores. Its scarcity during World War II, partly assured by bombing raids and daring Allied commando missions to destroy heavy-water production facilities, interfered critically with the German and Japanese nuclear programs.

It is believed the big bang produced the universe as much denser and hotter than it is now and made almost entirely of two main elements - hydrogen and helium. Deuterium itself was made only at a second stage of the beginning of the universe, namely through the collision of one neutron with one proton at a temperature of about one billion degrees; furthemore the two formed deuterons in turn stuck together into helium nuclei, which contain two protons and two neutrons. It is considered that during the formation of helium nuclei almost all the deuterons combined to form helium nuclei, leaving a tiny remant to be detected today so that only one in 10.000 deuterons remained unpaired.