A quadrilateral is a four-sided closed figure.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 sum of all angles of a quadrilateral is always 360˚. This is a special case of the -gon interior angle sum formula
A square, based on the criteria above, is both a rhombus and a rectangle and has all the properties of both of those shapes.
A quadrilateral may be cyclic. If opposite angles are supplementary, all its vertices will lie on the circumference a circle, called the circumcircle.