What is the X in X-mas?
tritium trit·i·um (trĭt'ē-əm, trĭsh'ē-)
A rare radioactive hydrogen isotope with atomic mass 3 and half-life 12.5 years, prepared artificially for use as a tracer and as a constituent of hydrogen bombs. Also called hydrogen-3.
A radioactive isotope of hydrogen whose nucleus has one proton and two neutrons with atomic mass of about 3 and a half life of 12.5 years. Tritium is rare in nature but can be made artificially in nuclear reactions. It is used in thermonuclear weapons and luminescent paints, and sometimes as a tracer. See more at hydrogen.