Heavy water is water that contains heavy hydrogen , also known as deuterium in place of regular hydrogen. It can also be written as D₂O.

Heavy water D₂O is used as a moderator in nuclear reactors and in exchange reactions for the study of reaction mechanisms. In nuclear fission reactors, the neutrons must be slowed down to ensure an effective fission chain reaction occurs. This process of slowing neutrons down is known as moderation, and the material that slows down these neutrons is known as a neutron moderator. Heavy water acts as a moderator.