A quadrilateral is a plane figure bounded by four straight sides.
This quadrilateral is called a convex quadrilateral because none of its four interior angles is a reflex angle.
This quadrilateral is called a non-convex quadrilateral because one of its interior angles is a reflex angle.
A quadrilateral has two diagonals that join opposite vertices.
Both diagonals of a convex quadrilateral are inside the quadrilateral.
One of the diagonals of a non-convex quadrilateral is outside the quadrilateral.