A quadrilateral is the polygon with four sides. Unlike the triangle, which is always convex, a quadrilateral can also “cave in” on itself.
Definition — Quadrilateral
A quadrilateral is a polygon with 4 sides and 4 interior angles.
Unlike triangles (which are always convex figures), quadrilaterals can also be non-convex: there exist two points inside the figure such that the segment joining them is not entirely inside the figure.
A convex quadrilateral and a non-convex one (concave): in the latter a vertex “caves in” towards the interior.
Links
Topics: Euclidean geometry
Concepts: Polygon · Quadrilateral