A polygon is a two-dimensional (flat) shape made up of a series of line segments joined end-to-end. Each of these segments comprises a side or edge of the polygon. The points where the sides meets are its vertices.
Polygons are used extensively in different branches of math to model objects, paths and spaces.
Commonly used polygons include:
Rectangles, rhombi, pentagons, hexagons and, of course... triangles