A cyst is an abnormal sac or cavity that contains liquid or semisolid material enclosed by a membrane. Cysts commonly occur in the sebaceous, or oil, glands of the skin, ovaries, sweat glands, esophagus, or breasts. Sebaceous cysts, also called whiteheads, occur when the oil glands are blocked and secretions from them cannot escape. When the ducts of sweat or mammary glands are blocked, retention cysts may develop. Most cysts are harmless and may be left alone. If a large cyst presses against adjacent tissue, it may be removed surgically.