Water composed of heavy isotopes of hydrogen or oxygen, or of both; esp. deuterium oxide, D₂O, which is water, composed of ordinary oxygen and deuterium. It is a colorless liquid, which forms hexagonal crystals on freezing. Its physical properties differ from those of normal water.

Deuterium oxide D₂O, occurs to small extent (about 0.003% by weight) in natural water, from which it can be separated by fractional distillation or by electrolysis.

It is useful in the nuclear industry because of its ability to reduce the energies of fast neutrons to thermal energies and because its absorption cross section is lower than that of hydrogen and consequently it does not appreciably reduce the neutron flux.

In the laboratory it is used for labellin other molecules for studies of reaction mechanisms.