Bursa: A closed, fluid-filled sac that functions as a gliding surface to reduce friction between tissues of the body. When a bursa becomes inflamed, the condition is known as bursitis. QUESTION The term arthritis refers to stiffness in the joints. See Answer From Arthritis Resources Inside the Visit With a RheumatologistTreating Psoriatic Disease With BiologicsRA and Your Joints Featured Centers Good and Bad Foods for PsoriasisVideo: Getting Personal on Life With MS Health Solutions From Our Sponsors Shot-Free MS Treatment Your Child and COVID-19