A rigid rotor is a 3-dimensional rigid body, such as a top -a children's toy. To describe the orientation of a 3-dimensional object with respect to a 3-dimensional coordinate system three angles are required. If the angles do not vary in time, the rigid body is standing still; when the angles vary in time the rigid body is rotating and is referred to as a rigid rotor (also known as rigid rotator).

A special rigid rotor is the linear rotor which is a 2-dimensional object (a pencil), requiring two angles to describe its orientation. Sometimes one considers rotation of a single particle around an axis fixed in space, then one angle is sufficient to describes the system fully.