What is the term for a closed sac or pouch that contains gas, fluid, semi-fluid, or solid material?

The term for a closed sac or pouch that contains gas, fluid, semi-fluid, or solid material is "cyst." Cysts are typically characterized by their encapsulated nature, which distinguishes them from other similar structures. They can develop in various tissues and organs and may contain a variety of substances, including air, pus, or other materials, depending on their type and the underlying pathology.

In medical terminology, a cyst is often benign, but it can still cause issues depending on its size and location within the body. This aspect of being a closed structure filled with material is what sets cysts apart from conditions like abscesses, which are collections of pus due to infection, and granulomas, which are small areas of inflammation that can arise due to chronic infection or foreign bodies. A fistula, on the other hand, is an abnormal connection between two body parts, typically resulting from disease or injury, and does not have the closed sac characteristic associated with cysts.