The Collaborative International Dictionary of English v.0.48:

Fission \Fis"sion\, n. [L. fissio. See Fissure.]
   1. A cleaving, splitting, or breaking up into parts.
      [1913 Webster]

   2. (Biol.) A method of asexual reproduction among the lowest
      (unicellular) organisms by means of a process of
      self-division, consisting of gradual division or cleavage
      of the into two parts, each of which then becomes a
      separate and independent organisms; as when a cell in an
      animal or plant, or its germ, undergoes a spontaneous
      division, and the parts again subdivide. See
      Segmentation, and Cell division, under Division.
      [1913 Webster]

   3. (Zool.) A process by which certain coral polyps,
      echinoderms, annelids, etc., spontaneously subdivide, each
      individual thus forming two or more new ones. See
      Strobilation.
      [1913 Webster]

   4. (Physics) The act or process of disintegration of an
      atomic nucleus into two or more smaller pieces; called
      also nuclear fission. The process may be spontaneous or
      induced by capture of neutrons or other smaller nuclei,
      and usually proceeds with evolution of energy.
      [PJC]

WordNet (r) 3.0 (2006):

fission
    n 1: reproduction of some unicellular organisms by division of
         the cell into two more or less equal parts
    2: a nuclear reaction in which a massive nucleus splits into
       smaller nuclei with the simultaneous release of energy [syn:
       fission, nuclear fission]