I. (ˈ)dī|agən ə l, -aig-, -g( ə )n ə l, -gnəl adjective
Etymology: Latin diagonalis, from Greek diagōnios from angle to angle (from dia- + -gōnios, from gōnia angle) + Latin -alis -al; akin to Greek gony knee — more at knee
1.
a. : joining two nonadjacent vertices of a rectilinear or polyhedral figure : running across from corner to corner
b. : passing through two nonadjacent edges of a polyhedron
a diagonal plane
2.
a. : inclined obliquely from a reference line (as the vertical or an axis)
wood with a diagonal grain
a map with a number of diagonal lines
a diagonal white cross on a blue ground
b. : having diagonal markings or parts
a diagonal weave
3. crystallog
a. : having reference to certain axes of the isometric system which are the intersections between the principal and the secondary planes of symmetry
b. : having reference to the directions bisecting the angles between lateral axes in the tetragonal and hexagonal systems
• di·ag·o·nal·ly -gən ə l]ē, -gnəl], ]i\ adverb
II. noun
( -s )
1. : a straight line joining any two nonadjacent vertices of a polygon or any two vertices of a polyhedron not in the same face
2.
a. : a diagonal direction : a diagonal row, arrangement, or pattern
water bugs skated hither and thither in apparently purposeless diagonals — S.E.White
neckties with colorful diagonals
b. : a twill weave : a twilled fabric especially of wool
c. : a line of squares running obliquely across a chessboard or checkerboard
one bishop moves along white diagonals
d. : something lying in a diagonal position (as in an inclined plane)
its gable wall rises from the falling diagonal of the ground — American Guide Series: Maryland
3. : the secondary mirror in a Newtonian reflecting telescope that is used to bring the focus to the side of the tube and is usually a flat mirror but sometimes a totally reflecting 45 degrees prism
4. : the symbol / used especially to denote “or” (as in and/or ), “and or” (as in straggler/deserter form ), “per” (as in feet/second ), “in” or “of” (as in U.S. Embassy/Paris ), “shilling” (as in 6/8 d ), or “for” (as in 2/39) to indicate division (as in birth/death ratio ) or the end of a line of verse or of a display line when quoted in running text, to separate terms of quantity (as in 5 tons /7 cwts /57 lbs ) or the figures of a date (as in 1/9/56), or to enclose phonemic rather than phonetic symbols — called also oblique, scratch comma, separatrix, slant, slash, slash mark, solidus, virgule
5. : an inclined member of a truss or bracing system excepting the end post of a truss and the top chord of a roof truss whose top chords are inclined