A tear of a tendon that connects muscles on the shoulder blade to the proximal humerus (the ball of the shoulder). It is a common source of shoulder pain.
About the Rotator Cuff
The rotator cuff tendon is a connection of three shoulder blade muscles to the proximal humerus. The muscles are the supraspinatus, the infraspinatus, and the teres minor. These three muscles blend together to form one continuous tendon that attaches to the proximal humerus (the ball of the shoulder) at an area of the proximal humerus called the greater tuberosity. Many popular anatomy drawings depict the rotator cuff tendon as if it is three separate tendons. This is not the case. This tendon is not distinguishable as three distinct components by either direct inspection or microscopic inspection.
My experience with inspecting the rotator cuff tendon at surgery is that in most males it is about 2-1/2 or 3 inches wide and about 3/16 inch thick. The thickness varies with the overall stature of the patient. The thickness also varies with age.