What does the medical term "cyst" refer to?

The term "cyst" specifically refers to a sac-like structure that can be filled with fluid, air, or other substances, and it is commonly associated with specific organs, including the bladder. In the context of the urinary system, a cyst can often refer to a cystic structure in the bladder, which may be involved in various medical conditions, such as infections or tumors. This term is typically used in diagnosing and describing conditions that involve swellings or cavities that are either benign or pathological in nature.

The other options listed pertain to different anatomical structures. While a kidney is responsible for filtering blood and producing urine, and stones or calculi refer to solid formations that can occur in organs such as the kidneys or bladder, and "night" does not have a medical correlation in this context. Therefore, the association of "cyst" with the bladder correctly relates to its definition and common clinical usage.