Final answer:
The bursa is the fluid-filled sac found in synovial joints that reduces friction between the skin and bone, providing a cushion to allow for smooth movement.
Step-by-step explanation:
The fluid-filled sac found in synovial joints that serves to reduce friction between skin and bone is the bursa.
Bursae are thin connective tissue sacs filled with lubricating liquid, typically located near body joints. They serve a crucial role in reducing friction and allowing for smooth movement over the bone. These sacs are found in various specific locations, such as subcutaneous bursae, which are situated between the skin and the underlying bone, submuscular bursae, located between a muscle and bone or between adjacent muscles, and subtendinous bursae, which are placed between a tendon and a bone.
Examples of bursae include the prepatellar bursa over the kneecap, the olecranon bursa at the elbow, and the subacromial bursa in the shoulder.