Final answer:
The articular capsule is the sac-like envelope that encloses the cavity of a synovial joint.
Step-by-step explanation:
The sac-like envelope which encloses the cavity of a synovial joint is called the articular capsule. The articular capsule is a fibrous connective tissue structure that is attached to each bone just outside the area of the bone's articulating surface. It forms the walls of the joint cavity and allows for smooth movements between the adjacent bones.