Inflammation (swelling, pain, and warmth) of a bursa. A bursa is a flat, fluid-filled sac found between a bone and a tendon or muscle. It forms a cushion to help the tendon or muscle slide smoothly over the bone. Bursitis may be caused by long-term overuse, trauma, rheumatoid arthritis, gout, or infection. It usually affects the shoulder, knee, elbow, hip, or foot.