within the mammalian body, any small pouch or sac between tendons (tendon), muscles (muscle), or skin and bony prominences at points of friction or stress. The bursas are classified by type as adventitious, subcutaneous, or synovial. Adventitious, or accidental, bursas arise in soft tissues as a result of repeated subjections to unusual shearing stresses, particularly over bony prominences. Subcutaneous bursas ordinarily are ill-defined clefts at the junction of subcutaneous tissue and deep fasciae (sheets of fibrous tissue); these bursas acquire a distinct wall only when they become abnormal, and they are sometimes classified as adventitious. Synovial bursas are thin-walled sacs that are interposed between tissues such as tendons, muscles, and bones (bone) and are lined with synovial membrane. In humans a majority of synovial bursas are located near the large joints (joint) of the arms and legs.

A bunion is an adventitious bursa that develops on the inner side of the base of the big toe in association with hallux valgus (deviation of the first toe such that it lies on top of or below the other toes). Wearing narrow, pointed shoes is a major contributory factor. Mild cases are relieved by use of proper shoes and foot care, but surgery may be necessary for correction of severe deformities.

Any type of inflammation of the bursas is called bursitis. The cause of most cases of bursitis appears to be local mechanical irritation, although bursas may also be involved along with the joints and tendon sheaths in rheumatoid arthritis and gout. Diseases of the bursa also occur in domestic animals. Capped elbow and capped hock are examples of chronic bursitis in horses, resulting from lying on hard floors.