di·ag·o·nal / dīˈagənl/ (abbr.: diag.) • adj. (of a straight line) joining two opposite corners of a square, rectangle, or other straight-sided shape. ∎  (of a line) straight and at an angle; slanting: a tie with diagonal stripes.• n. a straight line joining two opposite corners of a square, rectangle, or other straight-sided shape. ∎  Math. the set of elements of a matrix that lie on a line joining two opposite corners. ∎  a slanting straight pattern or line: the bars of light made diagonals across the entrance tiles can be laid on the diagonal. ∎  Chess a slanting row of squares whose color is the same.DERIVATIVES: di·ag·o·nal·ly adv.

diagonal XVI. — L. diagōnālis, f. Gr. diagṓnios, f. DIA- + gōníā angle; see -AL 1.

diagonal. See buttress and vault.