cyst, abnormal sac in the body, filled with a fluid or semisolid and enclosed in a membrane. Cysts can be congenital but are usually acquired, the most common locations being the skin and the ovaries. Sebaceous cysts of the skin, known as blackheads or whiteheads, occur when dirt or other material blocks the oil glands of the skin, preventing secretions from escaping. Retention cysts develop in glandular organs when ducts are blocked, commonly in kidney tubules, mammary glands, and sweat glands. Most cysts can be aspirated for treatment and/or cytology (diagnostic purposes); often cysts require surgical removal.