Pronunciation: (dī-ag'u-nl, -ag'nl),[key] —adj. 1. Math. a. connecting two nonadjacent angles or vertices of a polygon or polyhedron, as a straight line.
b. extending from one edge of a solid figure to an opposite edge, as a plane.
2. having an oblique direction.
3. having oblique lines, ridges, markings, etc.
—n. 1. a diagonal line or plane.
3. a diagonal row, part, pattern, etc.
4. Manège.(of a horse at a trot) the foreleg and the hind leg, diagonally opposite, which move forward simultaneously.
5. See diagonal cloth. 6. Math.a set of entries in a square matrix running either from upper left to lower right (main diagonal or principal diagonal) or lower left to upper right (secondary diagonal). 7. Chess.one of the oblique lines of squares on a chessboard: He advanced his bishop along the open diagonal.