In 3D modeling, a polygon is a multi-sided surface representation of a physical surface. A polygon can have three or more sides. A 3D model is formed from many polygons.
Four-sided and three-sided polygons
A polygon is always flat. A curved surface is formed by using many small polygons to form a gradual curve. To make a surface appear curved when viewed close-up, more polygons are needed.
A polygon is made up of one or more faces. Technically, a face is a three-sided polygon, but in practice these terms are used interchangeably.
A polygon should not be confused with a shape. A shape is just a line with no surface for rendering. A polygon represents a physical surface that can be rendered.