Source : Webster's Revised Unabridged Dictionary (1913)

Basal \Ba"sal\, a.
   Relating to, or forming, the base.

   {Basal cleavage}. See under {Cleavage}.

   {Basal plane} (Crystallog.), one parallel to the lateral or
      horizontal axis.

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

   {Prismatic cleavage}, cleavage parallel to a vertical prism.