Definition of cyst

• n. - A pouch or sac without opening, usually membranous and containing morbid matter, which is accidentally developed in one of the natural cavities or in the substance of an organ.
• n. - In old authors, the urinary bladder, or the gall bladder.
• n. - One of the bladders or air vessels of certain algae, as of the great kelp of the Pacific, and common rockweeds (Fuci) of our shores.
• n. - A small capsule or sac of the kind in which many immature entozoans exist in the tissues of living animals; also, a similar form in Rotifera, etc.
• n. - A form assumed by Protozoa in which they become saclike and quiescent. It generally precedes the production of germs. See Encystment.