The term for a two-dimensional shape with three sides is a triangle. Triangles are one of the fundamental shapes in geometry and can be classified based on their sides and angles. They can be categorized into three types: equilateral (all sides and angles are equal), isosceles (two sides and angles are equal), and scalene (all sides and angles are different). The angles of a triangle always add up to 180 degrees, which is a key principle in mathematics. Triangles are widely studied due to their properties and applications in various fields, including architecture, engineering, and the arts.