The Collaborative International Dictionary of English v.0.48:

tritium \trit"i*um\ (tr[i^]t"[i^]*[u^]m), n. (Chem.)
   A radioactive isotope of hydrogen having one proton and two
   neutrons in the nucleus. It decays spontaneously to Helium-3
   by the emission of an electron (beta ray), with a half-life
   of 12.3 years. Symbol 1H3. Atomic weight 3.01605 (C-12 =
   12.0000). It is one of the radioisotopes commonly used to
   label chemical compounds for use as tracers in biochemistry
   and chemistry. It is also used as one of the fusionable
   components of a hydrogen bomb.
   [PJC]

WordNet (r) 3.0 (2006):

tritium
    n 1: a radioactive isotope of hydrogen; atoms of tritium have
         three times the mass of ordinary hydrogen atoms