(Nucl Phys; Symb: T ) A radioactive isotope of hydrogen with two neutrons and one proton in the nucleus, which is called a triton. It is man-made and is heavier than deuterium (heavy hydrogen). Also called: Hydrogen-Three. Cf: Deuterium. See: Hydrogen. Note: Tritium is used in industrial thickness gauges and as a label in experiments in chemistry and biology