cyst

a small, usually benign, TUMOUR containing fluid (or soft secretions) within a membranous sac. Examples are SEBACEOUS CYSTS , cysts in the breasts (caused by blocked milk ducts), and ovarian cysts, which may be large and contain a clear, thick liquid. Dermoid cysts are congenital and occur at sites in the body where embryonic clefts have closed up before birth. These may contain fatty substances, hair, skin, fragments of bone and even teeth. Hydatid cysts are a stage in the life cycle of certain parasites (tapeworm) and may be found in humans, especially in the liver.