Source : Webster's Revised Unabridged Dictionary (1913)

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

   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}.

   3. (Zo["o]l.) A process by which certain coral polyps,
      echinoderms, annelids, etc., spontaneously subdivide, each
      individual thus forming two or more new ones. See
      {Strobilation}.

Source : WordNet®

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