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, "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").