The Collaborative International Dictionary of English v.0.48:

nuclear fusion \nuclear fusion\ n.
   A type of nuclear reaction in which atomic nuclei combine to
   form more massive nuclei with the simultaneous release of
   energy.

   Syn: fusion, nuclear fusion reaction.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

nuclear fusion
    n 1: a nuclear reaction in which nuclei combine to form more
         massive nuclei with the simultaneous release of energy
         [syn: fusion, nuclear fusion, nuclear fusion
         reaction]