Inflammation of the fluid-filled sac (bursa) that lies between a tendon and skin, or between a tendon and bone. The condition may be acute or chronic. Bursae are fluid-filled cavities near joints where tendons or muscles pass over bony projections. They assist movement and reduce friction between moving parts. Bursitis can be caused by chronic overuse, trauma, rheumatoid arthritis, gout, or infection. Sometimes the cause cannot be determined. Bursitis commonly occurs in the shoulder, knee, elbow, and hip. Other areas that may be affected include the Achilles tendon and the foot.