A quadrilateral (also called a tegragon or quadrangle) is a figure made up of four coplanar segments that have four non-colinear points as their endpoints and that intersect only at those four points.
Two sides of a quadrilateral that have a common end point are called consecutive sides. Two sides of a quadrilateral that don't have a common end point are called opposite sides. The segment that connects a pair of opposite vertices is called a diagonal.
In the right hand figure above we have the following examples: