A shoulder separation is an injury to the acromioclavicular joint on the top of the shoulder. The shoulder joint is formed at the junction of three bones: the collarbone (clavicle), the shoulder blade (scapula), and the arm bone (humerus). The scapula and clavicle form the socket of the joint, and the humerus has a round head that fits within this socket.
Source: Verywell Health