Definition of "diagonal"

(geometry) Joining two nonadjacent vertices (of a polygon or polyhedron).  examples

Having slanted or oblique lines or markings.  examples

Having a slanted or oblique direction.  quotations  examples

Of or related to the cater-corner (diagonally opposite) legs of a quadruped, whether the front left and back right or front right and back left.  examples

(geometry) A line joining non-adjacent vertices of a polygon.  examples

Anything forming or resembling such a line, particularly:

1. (geometry) A line or plane at an oblique angle to another.  examples

2. (fashion) A line or cut across a fabric at an oblique angle to its sides.  examples

3. (typography, uncommon) Synonym of slash ⟨/⟩.  quotations  examples

   Quotations

   • Initial inquiries among professional typists uncover names like slant, slant line, slash, and slash mark. Examination of typing instruction manuals discloses additional names such as diagonal and diagonal mark, and other sources provide the designation oblique.

     1965, Dmitri A. Borgmann, Language on Vacation, page 240