Final answer:
A thin layer of tissue covering a structure or cavity is known as a tissue membrane, which includes types such as epithelial, connective, and serous membranes, as well as mucous membranes that line exposed internal passageways.
Step-by-step explanation:
The term for a thin layer of tissue covering a structure or cavity is called a tissue membrane. These membranes can come in various forms, such as epithelial membranes, which line the skin, cover internal organs, and line cavities and tubes within the body; and connective tissue membranes, which include structures such as synovial membranes in joint cavities.
Additionally, a specific type called a serous membrane, or serosa, covers the walls and organs within the thoracic and abdominopelvic cavities, consisting of a parietal layer lining the body cavity and a visceral layer covering the organs, with a fluid-filled space between them.
A special kind of tissue membrane called mucous membrane is lined with mucous to protect tissues that are exposed to the outside environment. This includes many of the internal passageways of the body.