: a closed sac having a distinct membrane and developing abnormally in a body cavity or structure
: a body resembling a cyst: as a: a capsule formed about a minute organism going into a resting or spore stage; also: this capsule with its contents b: a resistant cover about a parasite produced by the parasite or the host—compare hydatid 2a
Enclosed sac within body tissues. It has a distinct membrane and generally contains liquid. Most cysts are benign, but several kinds may be malignant or precancerous. Benign cysts often press on nearby organs and require removal. Formed by overproduction of epithelium (surface tissue of anatomical structures), cysts may become detached from surrounding structures and move freely. They can contain natural secretions, abnormal breakdown products, or, in infections, bacteria, larval parasites, and microbial products. Some organs, including the kidney, liver, and breast, can become filled with cysts as a result of cystic diseases that may be dangerous or may hide more serious diseases. See alsotumour.