The Collaborative International Dictionary of English v.0.48:

nonparallel \nonparallel\ adj.
   1. not parallel; -- of lines or linear objects. Opposite of
      parallel. [Narrower terms: bias, catacorner,
      cata-cornered, catercorner, cater-cornered, catty-corner,
      catty-cornered, diagonal, kitty-corner, kitty-cornered,
      oblique, skew, skewed, slanted ; {crossed, decussate,
      intersectant, intersecting; cross-grained ; {diagonal;
      orthogonal, orthographic, rectangular, right-angled ;
      {right, perpendicular; angled ; {convergent] Also See:
      convergent, divergent, diverging.
      [WordNet 1.5]

   2. (Computers) Not using parallel processing; -- of
      computers. [Narrower terms: serial] PJC]

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]

The Collaborative International Dictionary of English v.0.48:

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.
      [1913 Webster]

   2. (Engin.) A member, in a framed structure, running
      obliquely across a panel.
      [1913 Webster]

   3. A diagonal cloth; a kind of cloth having diagonal stripes,
      ridges, or welts made in the weaving.
      [1913 Webster]

WordNet (r) 3.0 (2006):

diagonal
    adj 1: 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, diagonal, slanted, slanting, sloped,
       sloping]
    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: diagonal, 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, diagonal,
       stroke, separatrix]