A polygon is a closed two-dimensional shape. It is a simple curve that is made up of straight line segments. It usually has three sides and three corners or more.
It could also be referred to as 'A closed plane figure bound by three or more straight line segments'. It has a number of sides, also called edges. A square is a polygon because it has four sides. The smallest possible polygon in a Euclidean geometry or "flat geometry" is the triangle, but on a sphere, there can be a digon and a henagon.