What is the name for the fluid-filled space in synovial joints?

The term for the fluid-filled space in synovial joints is indeed the joint cavity. This cavity is essential for the function of synovial joints, as it contains synovial fluid, which is crucial for lubricating the joint surfaces, reducing friction, and providing nourishment to the articular cartilage.

The presence of the joint cavity allows for a range of motion between the articulating bones, making movements smoother and more efficient. The synovial fluid also acts as a shock absorber during weight-bearing activities, thus protecting the joint structures from damage.

Other structures mentioned, such as the synovial capsule, articular cartilage, and ligamentous sheath, play significant roles in the stability and functionality of the joint, but they do not refer to the fluid-filled space itself. The synovial capsule encases the joint, the articular cartilage covers the ends of the bones providing a smooth surface for articulation, and the ligamentous sheath helps stabilize the joint but does not contain the fluid.