Bursitis is inflammation of a bursa. A bursa is an enclosed soft tissue cavity, sometimes referred to as a sac or pad, filled with fluid that functions as a gliding surface to reduce friction between tissues of joints. There are many burs, plural for bursa, throughout the body located adjacent to the tendons near the large joints, such as the shoulder, elbow, hip, knee, and Achilles tendon (attaches calf muscles at the heal).

Bursitis can result from injury, infection or underlying inflammatory conditions. Patients suffering from bursitis experience swelling, tenderness, and pain during movement of the affected joint.