A cyst is a term used to describe a capsule or pouch that forms in the body. It can occur in various parts of the body, such as the skin, bones, or even internally, and is typically filled with fluid, air, or semi-solid material. The formation of a cyst can be caused by a variety of factors, including infection, injury, or a blockage in the ducts or glands. Therefore, the answer "capsule or pouch" accurately describes the nature of a cyst.