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]