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 origin of the word "quadrilateral" is the two Latin words quadri, a variant of four, and latus, meaning "side."
Quadrilaterals are simple (not self-intersecting) or complex (self-intersecting), also called crossed. Simple quadrilaterals are either convex or concave.
The interior angles of a simple (and planar) quadrilateral ABCD add up to 360 degrees of arc, that is
This is a special case of the n-gon interior angle sum formula (n − 2) × 180°. In a crossed quadrilateral, the interior angles on either side of the crossing add up to 720°.
All convex quadrilaterals tile the plane by repeated rotation around the midpoints of their edges.
Read more about Quadrilateral: Convex Quadrilaterals – Parallelograms, Convex Quadrilaterals – Other, More Quadrilaterals, Special Line Segments, Notations in Metric Formulas, Bimedians, Trigonometric Identities, Maximum and Minimum Properties, Remarkable Points and Lines in A Convex Quadrilateral, Other Properties of Convex Quadrilaterals, Taxonomy