Deuterium is a stable isotope of hydrogen, consisting of one proton and one neutron in its nucleus, giving it a mass approximately twice that of regular hydrogen. It is often represented by the symbol D or ²H. Deuterium occurs naturally in small amounts, making up about 0.0156% of all hydrogen found in water and other compounds. Deuterium is used in various scientific applications, including nuclear fusion research and as a tracer in chemical reactions. It is also utilized in the production of heavy water, which is used as a moderator in certain types of nuclear reactors, such as those in Canada.