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. SLIDESHOW What Is Rheumatoid Arthritis (RA)? Symptoms, Treatment, Diagnosis See Slideshow From Arthritis Resources Inside the Visit With a RheumatologistTaking AS One Day at a TimeWorking With Your Hands With PsA Featured Centers What Are the Best PsA Treatments for You?Understanding Biologics 10 Things People With Depression Wish You Knew Health Solutions From Our Sponsors Shot-Free MS Treatment Your Child and COVID-19