Bursitis is the painful swelling of bursae. Bursae are fluid-filled sacs that cushion your tendons, ligaments, and muscles. When they work normally, bursae help the tendons, ligaments, and muscles glide smoothly over bone. But when the bursae are swollen, the area around them becomes very tender and painful. Trochanteric bursitis is swelling affecting the bursae of the hip.
Source: familydoctor.org