What type of joint is characterized by a thin layer of viscous fluid separating two cartilage-covered bone surfaces?

A synovial joint is distinguished by the presence of a synovial cavity, which is filled with synovial fluid. This fluid serves multiple functions, including lubricating the joint to reduce friction between the cartilage-covered bone surfaces, providing nutrients to the cartilage, and acting as a shock absorber during movement. The thin layer of viscous fluid enables smooth and flexible motion between the bones, which is crucial for activities such as walking, running, and grasping.

The other types of joints mentioned do not exhibit this characteristic. Fixed joints, also known as immovable joints, are connected by fibrous tissue and do not allow for movement. Hinge joints allow for movement in one plane but do not have a synovial cavity like synovial joints. Pivot joints allow for rotational movement around a single axis but also lack the fluid-filled cavity that defines synovial joints. Therefore, the defining feature of synovial joints is the presence of the synovial fluid, which is essential for their function and mobility.