Shoulder instability occurs when the shoulder (glenohumeral) joint does not remain in the correct alignment. This can occur when any of the outlying structures do not maintain stability to the ball-and-socket formation.

Due to abnormal looseness, the ball of the joint–which is the head of the upper arm, or humerus–may slide out of place. This is referred to as subluxation. If the shoulder joint shifts completely out of place, it is considered a shoulder dislocation.

Further Reading:

American Academy of Orthopaedic Surgeons: Chronic Shoulder Instability