Deuterium-depleted water (DDW), also known more ambiguously as light water, is water which has a lower concentration of deuterium than occurs naturally. Deuterium is a heavier isotope of hydrogen which has, in addition to its one proton, a neutron that roughly doubles the mass of the hydrogen atom.
In Vienna Standard Mean Ocean Water, deuterium occurs at a rate of 155.76 ppm. The production of heavy water involves isolating and removing deuterium within water. The by-product of this process is deuterium-depleted water.