The Collaborative International Dictionary of English v.0.48:

Cleavage \Cleav"age\, n.
   1. The act of cleaving or splitting.
      [1913 Webster]

   2. (Crystallog.) The quality possessed by many crystallized
      substances of splitting readily in one or more definite
      directions, in which the cohesive attraction is a minimum,
      affording more or less smooth surfaces; the direction of
      the dividing plane; a fragment obtained by cleaving, as of
      a diamond. See Parting.
      [1913 Webster]

   3. (Geol.) Division into lamin[ae], like slate, with the
      lamination not necessarily parallel to the plane of
      deposition; -- usually produced by pressure.
      [1913 Webster]

   Basal cleavage, cleavage parallel to the base of a crystal,
      or to the plane of the lateral axes.

   Cell cleavage (Biol.), multiplication of cells by fission.
      See Segmentation.

   Cubic cleavage, cleavage parallel to the faces of a cube.


   Diagonal cleavage, cleavage parallel to ta diagonal plane.


   Egg clavage. (Biol.) See Segmentation.

   Lateral cleavage, cleavage parallel to the lateral planes.


   Octahedral cleavage, Dodecahedral cleavage, or
   Rhombohedral cleavage, cleavage parallel to the faces of an
      octahedron, dodecahedron, or rhombohedron.

   Prismatic cleavage, cleavage parallel to a vertical prism.
      [1913 Webster]

WordNet (r) 3.0 (2006):

cleavage
    n 1: the state of being split or cleft; "there was a cleavage
         between the liberal and conservative members"
    2: the breaking of a chemical bond in a molecule resulting in
       smaller molecules
    3: (embryology) the repeated division of a fertilised ovum [syn:
       cleavage, segmentation]
    4: the line formed by a groove between two parts (especially the
       separation between a woman's breasts)
    5: the act of cleaving or splitting

Moby Thesaurus II by Grady Ward, 1.0:

74 Moby Thesaurus words for "cleavage":
   abscission, alienation, alpha decay, amputation, atom-chipping,
   atom-smashing, atomic disintegration, atomic reaction, atomization,
   beta decay, bombardment, breach, breach of friendship, break,
   breeding, bullet, butchering, chain reaction, chasm, chopping,
   cleft, cutting, dichotomy, disaffection, disfavor,
   disintegration series, disruption, dissociation, disunion,
   disunity, divergence, dividedness, division, enucleation,
   estrangement, exchange reaction, excision, falling-out, fission,
   fission reaction, gamma decay, ionization, laceration, mutilation,
   neutron reaction, nonreversible reaction, nuclear fission,
   nucleization, open rupture, photodisintegration, proton gun,
   proton reaction, recall of ambassadors, rending, resection,
   reversible reaction, rift, ripping, rupture, schism, scission,
   section, separation, severance, slashing, slicing, split,
   splitting, splitting the atom, stimulation, surgery, target,
   tearing, thermonuclear reaction