Meaning

noun (geometry) a straight line connecting any two vertices of a polygon that are not adjacent

noun a line or cut across a fabric that is not at right angles to a side of the fabric

noun an oblique line of squares of the same color on a checkerboard

originFrom Middle French diagonal, from Latin diagōnālis, from Ancient Greek διαγώνιος (diagṓnios, “from angle to angle”), from διά (diá, “across”) + γωνία (gōnía, “angle”).

Etymology from Wiktionary, CC BY-SA 3.0