Bursitis

Bursitis is an easily treatable disease caused by inflammation of a bursa. Bursae (plural form) are small fluid-filled sacs that function as cushions between bone and other moving tissues of the musculoskeletal system. It is estimated that there are over 150 bursae in the human body, with about 50 of them actually named. [1]. Although any bursa can get inflamed, the bursae of the larger joints are the ones most commonly affected. These include joints such as the shoulders, elbows, hips, knees, ankles, and feet.

Source reference: http://wiki.medpedia.com/Bursitis