The Collaborative International Dictionary of English v.0.48:
Cleavage \Cleav"age\, n.
1. The act of cleaving or splitting.
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.
3. (Geol.) Division into lamin[ae], like slate, with the
lamination not necessarily parallel to the plane of
deposition; -- usually produced by pressure.
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.
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.