What is defined as an abnormal growth that has a distinct membrane and is often filled with liquid?

A cyst is defined as an abnormal growth characterized by a distinct membrane that can be filled with liquid, semi-solid, or gaseous material. Cysts can develop in various tissues throughout the body and are often benign, meaning they are not cancerous. The fluid within a cyst can vary in composition, and the wall of the cyst provides a clear boundary separating it from surrounding tissues.

In contrast, a tumor is a mass of tissue that can be benign or malignant (cancerous) but does not typically have the same clear membrane surrounding it as a cyst does. An abscess is a localized collection of pus that occurs due to an infection and is usually accompanied by inflammation and pain; it does not have a distinct membrane like cysts do. A pustule is a small, inflamed, blister-like elevation on the skin that is filled with pus, commonly seen in conditions like acne; like abscesses, pustules do not have a protective membrane in the same way cysts do.

Thus, the defining characteristics of a cyst—its membrane and its often liquid-filled nature—make it the correct choice in this question.