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.
Other articles related to "diagonal":
... set X with itself, consisting of all pairs (x,x), is called the diagonal, and is the graph of the identity relation ... may be obtained by intersecting the graph of F with the diagonal ... In geometric studies, the idea of intersecting the diagonal with itself is common, not directly, but by perturbing it within an equivalence class ...