The Collaborative International Dictionary of English v.0.48:

nuclear fission \nuclear fission\ n.
   A nuclear reaction in which a heavy atomic nucleus splits
   into smaller nuclei with the simultaneous release of energy.

   Syn: fission.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

nuclear fission
    n 1: a nuclear reaction in which a massive nucleus splits into
         smaller nuclei with the simultaneous release of energy
         [syn: fission, nuclear fission]