Simple Definition:

Bursitis is a condition that involves inflammation [swelling] of the bursae [small, fluid-filled sacs] in the body. Bursae are located near joints and act as cushions (soft padding) between bones, muscles, tendons [tough bands of tissue that connect muscles to bones], and skin. Bursitis can cause pain, tenderness [sensitivity to touch], and swelling in the affected area. For example, a common type of bursitis is shoulder bursitis, which causes pain and limited range of motion in the shoulder joint.

Very Simple Definition:

Bursitis is a condition where the bursae, which are small sacs near joints that act as cushions, become swollen and cause pain. For example, shoulder bursitis makes the shoulder hurt and hard to move.