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Heavy water (deuterium oxide (2H 2O) or D 2O) is a form of water that contains a larger than normal amount of the hydrogen isotope deuterium (2H or D, also known as heavy hydrogen), rather than the common hydrogen-1 isotope (1H or H, also called protium) that makes up most of the hydrogen in regular water.
Heavy water molecules are heavy because they contain deuterium. Ordinarily, the nucleus of a hydrogen atom consists of a single proton, but deuterium is an isotope of hydrogen that contains one neutron as well as one proton, vastly increasing the weight of the element and altering its atomic properties.