A quadrilateral is a polygon that has four sides.
The figures below are all quadrilaterals.
opposite sides and angles of a quadrilateral
For a quadrilateral, opposite sides do not have a common vertex:
As can be seen in the figure above, line segments AB and CD are opposite sides of the quadrilateral, and do not share a common vertex. The same is true of line segments AD and BC.
For a quadrilateral, opposite angles do not have a common side:
As can be seen in the figure above, angle A and angle C are opposite angles in the quadrilateral. Angles B and D are also opposite angles, and do not share a common side.
See also polygon.