a radioactive isotope of hydrogen having an atomic weight of 3 and a half-life of c. 12.5 years: it decays by beta-particle emission and is used in thermonuclear bombs, thermonuclear fusion devices, as a radioactive tracer, etc.
ModL < Gr tritos, third + -ium
See tritium in American Heritage Dictionary 4
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.
Origin: From Greek tritos, third; see trei- in Indo-European roots