A supplementary angle is defined as two angles that add up to 180 degrees. For example, if one angle measures 120 degrees, the other must measure 60 degrees to be considered supplementary. This concept is commonly used in geometry and can be found in various shapes and figures. Supplementary angles can be adjacent, meaning they share a common side and vertex, or they can be separate. Understanding supplementary angles is essential for solving problems related to triangles, polygons, and other geometric figures, as they help in determining unknown angle measures.