The definition of diagonal is something with slanted lines or a line that connects one corner with the corner furthest away.(adjective)
An example of diagonal is a line going from the bottom left corner of a square to the top right corner.
See diagonal in Webster's New World College Dictionary
Origin: L diagonalis < Gr diagōnios < dia-, through + gōnia, an angle, corner: see knee
See diagonal in American Heritage Dictionary 4
Origin: Latin diagōnālis
Origin: , from Greek diagōnios, from angle to angle
Origin: : dia-, dia-
Origin: + gōniā, angle, corner; see genu-1 in Indo-European roots.
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