The Collaborative International Dictionary of English v.0.48:

Diagonal \Di*ag"o*nal\, a. [L. diagonalis, fr. Gr. ? from to
   angle; dia` through + ? an angle; perh. akin to E. knee: cf.
   F. diagonal.] (Geom.)
   Joining two not adjacent angles of a quadrilateral or
   multilateral figure; running across from corner to corner;
   crossing at an angle with one of the sides.
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   Diagonal bond (Masonry), herringbone work. See
      Herringbone, a.

   Diagonal built (Shipbuilding), built by forming the outer
      skin of two layers of planking, making angles of about
      45[deg] with the keel, in opposite directions.

   Diagonal cleavage. See under Cleavage.

   Diagonal molding (Arch.), a chevron or zigzag molding.

   Diagonal rib. (Arch.) See Cross-springer.

   Diagonal scale, a scale which consists of a set of parallel
      lines, with other lines crossing them obliquely, so that
      their intersections furnish smaller subdivisions of the
      unit of measure than could be conveniently marked on a
      plain scale.

   Diagonal stratification. (Geol.) Same as Cross bedding,
      under Cross, a.
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The Collaborative International Dictionary of English v.0.48:

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