What is the primary characteristic of a cyst?

A cyst is primarily defined by being a closed sac-like structure that is filled with fluid, semi-solid material, or gas. This characteristic distinguishes cysts from other types of growths or lesions. The fluid within a cyst can vary in consistency, depending on the type of cyst and the tissue of origin.

While inflammation can occur in a cyst, it is not a defining feature, as not all cysts are inflamed. Similarly, cysts can cause pain depending on their location and size, but pain is not an inherent quality of cysts themselves. The development of cysts in foreign tissue is not a primary characteristic either; cysts can arise from various tissues in the body, not necessarily only foreign tissues. Therefore, the defining characteristic that accurately describes what a cyst is, focuses on its composition, which is that it is filled with fluid.