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]

Moby Thesaurus II by Grady Ward, 1.0:

124 Moby Thesaurus words for "fission":
   ablate, abscission, accelerate, activate, alpha decay, amputation,
   atom-chipping, atom-smashing, atomic disintegration,
   atomic reaction, atomization, atomize, beta decay, bifurcate,
   bisect, bombard, bombardment, branch, break into pieces,
   break to pieces, break up, breeding, bullet, butchering, by two,
   catalysis, catalyst, chain reaction, chopping, cleavage, cleave,
   come apart, consume, corrode, crack up, crash, cross-bombard,
   crumble, crumble into dust, crunch, crush, cut in two,
   cut to pieces, cutting, decay, decompose, demolish, dialysis,
   dichotomize, dichotomy, diffuse, dimidiate, disintegrate,
   disintegration series, disjoin, disorganize, disperse, disrupt,
   dissociation, dissolve, divide, enucleation, erode,
   exchange reaction, excision, fall to pieces, fission reaction,
   fork, fragment, gamma decay, grind, halve, hydrolysis, hydrolyst,
   in half, ionization, laceration, make mincemeat of, mince, molder,
   mutilation, neutron reaction, nonreversible reaction,
   nuclear fission, nucleization, nucleize, photodisintegration,
   photolysis, proton gun, proton reaction, pulverize, ramify,
   rending, resection, reversible reaction, ripping, scatter,
   scission, section, severance, shatter, shiver, slashing, slicing,
   smash, smash the atom, smash up, splinter, split, split in two,
   splitting, splitting the atom, squash, squish, stimulation,
   subdivide, surgery, target, tearing, thermolysis,
   thermonuclear reaction, transect, waste away, wear away