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.
   [1913 Webster]

   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.
      [1913 Webster]