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### Definition of diagonal

diagonal - adj. 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; having an oblique or slanted direction; noun (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; an oblique line of squares of the same color on a checkerboard; a line or cut across a fabric that is not at right angles to a side of the fabric; (geometry) a straight line connecting any two vertices of a polygon that are not adjacent; a punctuation mark (/) used to separate related items of information.

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