main diagonal

noun Mathematics.

See under diagonal(def 9).


[dahy-ag-uh-nl, -ag-nl]


  1. connecting two nonadjacent angles or vertices of a polygon or polyhedron, as a straight line.
  2. extending from one edge of a solid figure to an opposite edge, as a plane.
having an oblique direction.
having oblique lines, ridges, markings, etc.


a diagonal line or plane.
a diagonal row, part, pattern, etc.
Manège. (of a horse at a trot) the foreleg and the hind leg, diagonally opposite, which move forward simultaneously.
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.

Origin of diagonal

1535–45; < Latin diagōnālis < Greek diagṓn(ios) from angle to angle (see dia-, -gon) + Latin -ālis -al1
Related formsdi·ag·o·nal·ly, adverbnon·di·ag·o·nal, adjective, nounnon·di·ag·o·nal·ly, adverb Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019

British Dictionary definitions for main diagonal



maths connecting any two vertices that in a polygon are not adjacent and in a polyhedron are not in the same face
slanting; oblique
marked with slanting lines or patterns


maths a diagonal line or plane
chess any oblique row of squares of the same colour
cloth marked or woven with slanting lines or patterns
something put, set, or drawn obliquely
another name for solidus (def. 1)
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
Derived Formsdiagonally, adverb

Word Origin for diagonal

C16: from Latin diagōnālis, from Greek diagōnios, from dia- + gōnia angle
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012

Word Origin and History for main diagonal



1540s (implied in diagonally), from Middle French diagonal, from Latin diagonalis, from diagonus "slanting line," from Greek diagonios "from angle to angle," from dia- "across" (see dia-) + gonia "angle," related to gony "knee" (see knee (n.)). As a noun, from 1570s.

Online Etymology Dictionary, © 2010 Douglas Harper

Science definitions for main diagonal




Connecting two nonadjacent corners in a polygon or two nonadjacent corners in a polyhedron that do not lie in the same face.


A diagonal line segment.
The American Heritage® Science Dictionary Copyright © 2011. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.