What is the definition of Saddle Joint?

Saddle Joint: In a saddle joint (also called a sellar joint or articulation by reciprocal reception) the opposing surfaces are reciprocally concave-convex, allowing for motion in 2 planes. Motion is similar to how a horseback rider can move in a saddle. Examples include the trapeziometacarpal joint of the thumb (1st carpometacarpal joint) and the sternoclavicular joint.