Principal-diagonal

noun
1.
Mathematics. See under diagonal (def 9).
adjective
1.
Mathematics.

connecting two nonadjacent angles or vertices of a polygon or polyhedron, as a straight line.
extending from one edge of a solid figure to an opposite edge, as a plane.

2.
having an oblique direction.
3.
having oblique lines, ridges, markings, etc.
noun
4.
a diagonal line or plane.
5.
virgule.
6.
a diagonal row, part, pattern, etc.
7.
Manège. (of a horse at a trot) the foreleg and the hind leg, diagonally opposite, which move forward simultaneously.
8.
diagonal cloth.
9.
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.
adjective
1.
(maths) connecting any two vertices that in a polygon are not adjacent and in a polyhedron are not in the same face
2.
slanting; oblique
3.
marked with slanting lines or patterns
noun
4.
(maths) a diagonal line or plane
5.
(chess) any oblique row of squares of the same colour
6.
cloth marked or woven with slanting lines or patterns
7.
something put, set, or drawn obliquely
8.
another name for solidus (sense 1)
9.
one front leg and the hind leg on the opposite side of a horse, which are on the ground together when the horse is trotting
diagonal
(dī-āg’ə-nəl)
Adjective Connecting two nonadjacent corners in a polygon or two nonadjacent corners in a polyhedron that do not lie in the same face.

Noun A diagonal line segment.