1. A freely moving joint in which a sphere on the head of one bone fits into a rounded cavity in the other bone; SYN. spheroid joint, cotyloid joint, enarthrodial joint, enarthrosis, articulatio spheroidea.
2. A joint that can rotate within a socket.
Joint allowing considerable movement in three dimensions, for instance the joint between the pelvis and the femur. To facilitate movement, such joints are rimmed with cartilage and lubricated by synovial fluid. The bones are kept in place by ligaments and moved by muscles.