Final answer:
A joint that allows motion in two planes is a biaxial joint, such as the metacarpophalangeal joint in the hand, which allows for bending and spreading of the fingers. These joints are part of the diarthrotic category, which is known for providing a wide range of motion.
Step-by-step explanation:
A joint that allows motion in two planes is classified as a biaxial joint. An example of a biaxial joint would be the metacarpophalangeal joint (knuckle joint) of the hand, which permits movement along one axis to produce bending or straightening of the finger, and movement along a second axis, which allows for the spreading of the fingers away from each other and bringing them together. Synovial joints, which include plane joints and ball-and-socket joints, are functionally classified as diarthroses and provide a wide range of motion. Biaxial joints form part of the diarthrotic category, defined by their freedom of movement, as opposed to uniaxial joints like the elbow which only allow movement in one plane, or multiaxial joints like the hip and shoulder, which allow movement in three planes.