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).
10.
Chess. one of the oblique lines of squares on a chessboard: He advanced his bishop along the open diagonal.
Origin: 1535–45; < L diagōnālis < Gk diagn(ios) from angle to angle (see dia-, -gon) + L -ālis-al1
a short oblique stroke (/) between two words indicating that whichever is appropriate may be chosen to complete the sense of the text in which they occur: The defendant and/or his/her attorney must appear in court.
2.
a dividing line, as in dates, fractions, a run-in passage of poetry to show verse division, etc.: 3/21/27; 3/4; Sweetest love I do not go/For weariness of thee.
Joining two vertices of a polyhedron not in the same face.
Having a slanted or oblique direction.
Having oblique lines or markings.
Relating to or being the front left and back right feet or the front right and back left feet of a quadruped.
n.
Mathematics A diagonal line or plane.
Something, such as a row, course, or part, that is arranged obliquely.
A fabric woven with diagonal lines.
A virgule.
[Latin diagōnālis, from Greek diagōnios, from angle to angle : dia-, dia- + gōniā, angle, corner; see genu-1 in Indo-European roots.] di·ag'o·nal·ly adv.
diagonal (dī-āg'ə-nəl) Pronunciation Key
Adjective
Connecting two nonadjacent corners in a polygon or two nonadjacent corners in a polyhedron that do not lie in the same face.
[dahy-ag-uh-nl, -ag-nl] 
ə
n