…(of a straight line) joining two opposite corners of a square, rectangle, or other straight-sided shape…
…denoting an element of a square matrix that is not on the diagonal running from the upper left to the lower right…
…a matrix having non-zero elements only in the diagonal running from the upper left to the lower right…
…the set of elements of a matrix that lie on the line joining the top left corner to the bottom right corner…
