Origin: 1500-1600 Latin diagonalis, from Greek diagonios from angle to angle
1 following a sloping angle: diagonal parking spaces2math, geometry a diagonal line is straight and joins two opposite corners of a flat shape that has three or more sides, or a solid shape with three or more sides → horizontal: Draw a diagonal line across the square. [Origin: 1500–1600 Latin diagonalis, from Greek diagonios from angle to angle]—diagonal noun [countable]—diagonally adverb