Source : Webster's Revised Unabridged Dictionary (1913)

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.

   {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.

Diagonal \Di*ag"o*nal\, n.
   1. A right line drawn from one angle to another not adjacent,
      of a figure of four or more sides, and dividing it into
      two parts.

   2. (Engin.) A member, in a framed structure, running
      obliquely across a panel.

   3. A diagonal cloth; a kind of cloth having diagonal stripes,
      ridges, or welts made in the weaving.

Source : WordNet®

diagonal
     adj 1: at an angle; especially connecting two nonadjacent corners
            of a plane figure or any two corners of a solid that
            are not in the same face; "a diagonal line across the
            page"
     2: having an oblique or slanted direction [syn: {aslant}, {aslope},
         {slanted}, {slanting}, {sloped}, {sloping}]

diagonal
     n 1: (geometry) a straight line connecting any two vertices of a
          polygon that are not adjacent
     2: a line or cut across a fabric that is not at right angles to
        a side of the fabric [syn: {bias}]
     3: an oblique line of squares of the same color on a
        checkerboard; "the bishop moves on the diagonals"
     4: (mathematics) a set of entries in a square matrix running
        diagonally either from the upper left to lower right entry
        or running from the upper right to lower left entry
     5: a punctuation mark (/) used to separate related items of
        information [syn: {solidus}, {slash}, {virgule}, {stroke},
         {separatrix}]