adjective

1. extending between the vertices of any two nonadjacent angles in a polygonal figure or between any two vertices not in the same face in a polyhedral figure; extending slantingly between opposite corners
2. moving or extending obliquely, esp. at a 45° angle; slanting
3. having slanting markings, lines, etc.

Etymology: L diagonalis < Gr diagōnios < dia-, through + gōnia, an angle, corner: see knee

noun

1. 1. a diagonal line or plane
   2. virgule
2. any diagonal course, row, order, or part
3. cloth woven with diagonal lines; twill