A bursa is a fluid-filled space lined by endothelium. They function to permit free movement between apposing tendon, bone and skin.

There are a number of well-defined bursae:

• olecranon bursa
• pre-patellar bursa
• infra-patellar bursa
• ischial bursa
• trochanteric bursa
• gluteofemoral bursa

Also, bursae may form at the sites of repeated friction between two layers: adventitious bursae.

Bursae may become inflamed or infected.