What is Joint?

Legal Definition
A joint or articulation (or articular surface) is the connection made between bones in the body which link the skeletal system into a functional whole. They are constructed to allow for different degrees and types of movement. Some joints, such as the knee, elbow, and shoulder, are self-lubricating, almost frictionless, and are able to withstand compression and maintain heavy loads while still executing smooth and precise movements. Other joints such as sutures between the bones of the skull permit very little movement (only during birth) in order to protect the brain and the sense organs. The connection between a tooth and the jawbone is also called a joint, and is described as a fibrous joint known as a gomphosis. Joints are classified both structurally and functionally.
-- Wikipedia
Legal Definition
United, not separate; as, joint action, or one which is brought by several persons acting together; joint bond, a bond given by two or more obligors.
-- Bouviers Law Dictionary
Legal Definition
United; combined; undivided; done by or against two or more unitedly; shared by or between two or more. A "joint" bond, note, or other obligation is one in which the obligors or makers (being two or more in number) bind themselves jointly but not severally, and which must therefore be prosecuted in a joint action against them nil. A "joint and several" bond or note is one in which the obligors or makers bind themselves both jointly and individually to the obligee or payee, and which may be enforced either by a joint action against them all or by separate actions against any one or more at the election of the creditor.
See also
-- Black's Law Dictionary
Legal Definition
United; sharing an interest; sharing liability.
-- Ballentine's Law Dictionary