A small fluid-filled sac that helps to reduce friction near a joint is called a:

A bursa is a small fluid-filled sac located in joints and other areas of the body where friction occurs, such as between bones and tendons or muscles. Its primary function is to cushion and facilitate smooth movement by reducing friction and allowing for easier gliding of tissues over one another during motion.

In contrast, a cyst is typically a closed sac containing fluid, semi-solid material, or gas, but it does not serve the same function as a bursa in reducing friction around joints. A tendon is a connective tissue that attaches muscle to bone, and while it plays a critical role in movement, it does not function as a friction-reducing sac. A ligament also serves a different purpose, connecting bones to other bones and providing stability to joints rather than lowering friction. Therefore, the bursa is specifically designed for the purpose of reducing friction, making it the correct answer.