Tritium, hydrogen isotope that is 3 times heavier than ordinary hydrogen. One of 3 hydrogen isotopes (the others are protium and deuterium), tritium contains 1 proton and 2 neutrons in its nucleus. It is used in the release of nuclear energy through the fusion process. A radioactive gas and a key component of the hydrogen bomb, it is also used in luminous paints and as a tracer. Helium is formed when tritium decays.