A **diagonal** is a line joining two nonconsecutive vertices of a polygon or polyhedron. Informally, any sloping line is called diagonal. The word "diagonal" derives from the ancient Greek διαγώνιος *diagonios*, "from angle to angle" (from διά- *dia-*, "through", "across" and γωνία *gonia*, "angle", related to *gony* "knee"); it was used by both Strabo and Euclid to refer to a line connecting two vertices of a rhombus or cuboid, and later adopted into Latin as *diagonus* ("slanting line").

In mathematics, in addition to its geometric meaning, a diagonal is also used in matrices to refer to a set of entries along a diagonal line.

Read more about Diagonal: Non-mathematical Uses, Polygons, Matrices, Geometry

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