What is defined as a sac containing a gaseous, liquid, or semi-solid substance?

A cyst is defined as a sac-like structure that can contain a gaseous, liquid, or semi-solid substance. These formations are usually encapsulated and can occur in various tissues in the body. Cysts can form as a result of infections, blockages of ducts, or even genetic conditions. They may vary in size and can be found anywhere in the body, including the skin, organs, and tissues.

The term "tumor" refers more broadly to an abnormal mass of tissue, which may be cancerous or benign, but does not specify that it has a sac-like structure containing fluid or other substances. "Lesion" is a general term used to describe any abnormal change in the structure of an organ or tissue due to injury or disease, and does not imply the presence of a sac. An "abscess" specifically refers to a collection of pus that occurs due to infection, which is different from the more generalized definition of a cyst. Therefore, a cyst is the correct answer as it specifically fits the definition provided in the question.