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").