Mathematics. a set of entries in a square matrix running either from upper left to lower right (main diagonal or principal diagonal) or lower left to upper right (secondary diagonal).

Chess. one of the oblique lines of squares on a chessboard: He advanced his bishop along the open diagonal.

SEE MORESEE LESS

Origin of diagonal

1535–45; < Latindiagōnālis < Greekdiagṓn(ios) from angle to angle (see dia-, -gon) + Latin-ālis-al^{1}

Related formsdi·ag·o·nal·ly, adverbnon·di·ag·o·nal, adjective, nounnon·di·ag·o·nal·ly, adverb