High Quality Content by WIKIPEDIA articles! In Euclidean plane geometry, a quadrilateral is a polygon with four sides (or `edges`) and four vertices or corners. Sometimes, the term quadrangle is used, by analogy with triangle, and sometimes tetragon for consistency with pentagon (5-sided), hexagon (6-sided) and so on. The word quadrilateral is made of the words quad (meaning "four") and lateral (meaning "of sides").