An exterior angle is formed when one side of a polygon is extended, creating an angle outside the shape. In a triangle, for example, the exterior angle is equal to the sum of the two opposite interior angles. This property helps in solving various geometric problems and understanding the relationships between angles. Exterior angles can be found in various polygons, including triangles, quadrilaterals, and pentagons. The sum of the exterior angles of any polygon is always 360 degrees, regardless of the number of sides. This consistent rule makes exterior angles a fundamental concept in geometry.