In geometry, a diagonal is a line segment joining two vertices of a polygon or polyhedron, when those vertices are not on the same edge. Informally, any sloping line is called diagonal. The word diagonal derives from the ancient Greek διαγώνιος diagonios,[1] “from angle to angle” (from διά- dia-, “through”, “across” and γωνία gonia, “angle”, related to gony “knee”); it was used by both Strabo[2] and Euclid[3] to refer to a line connecting two vertices of a rhombus or cuboid,[4] and later adopted into Latin as diagonus (“slanting line”).

In matrix algebra, a diagonal of a square matrix is a set of entries extending from one corner to the farthest corner.