deuterium (dōtērˈēəm) [key], isotope of hydrogen with mass no. 2. The deuterium nucleus, called a deuteron, contains one proton and one neutron. Deuterium is also called heavy hydrogen, and water in which the hydrogen atoms are deuterium is called heavy water (deuterium oxide, D2O). Deuterons are sometimes used in particle accelerators, and heavy water is used in "swimming pool" nuclear reactors as a moderator.