Which of the following joint types is defined by both bones having concave-convex surfaces that interlock?

The type of joint characterized by the interlocking of concave-convex surfaces of both bones is the saddle joint. In a saddle joint, each bone has both a concave and a convex surface, allowing for a greater range of motion compared to other joint types. This unique design facilitates movements such as flexion, extension, abduction, and adduction while allowing some degree of rotation, making it particularly versatile.

For example, the carpometacarpal joint of the thumb is a classic example of a saddle joint, enabling the thumb's opposition and grasping capability. This interlocking nature of the concave and convex surfaces provides stability while allowing for significant mobility in multiple directions, which is distinct from the single-axis movements permitted by hinge joints, the rotational movement in pivot joints, or the multi-directional but spherical movement in ball and socket joints.