cyst

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun An abnormal membranous sac in the body containing a gaseous, liquid, or semisolid substance.
• noun A sac or vesicle in the body.
• noun Biology A small capsulelike sac that encloses certain organisms in their dormant or larval stage.
• noun Botany A thick-walled resting spore, as in certain algae or fungi.

from The Century Dictionary.

• noun In anatomy, a bladder; a large vesicle.
• noun In pathology, a bladder-like bag or vesicle in animal bodies which includes morbid matter.
• noun In zoology, a hydatid; a cystic worm, or encysted state of a tapeworm.
• noun In cryptogamic botany, a cell or cavity, usually inclosing other cells or reproductive bodies, as an envelop inclosing a group of diatoms or desmids, or a cell containing an antherozoid; in certain algæ, a sporecase. See coniocyst.
• noun Sometimes, improperly, cist.
• noun The resistant coating formed by the parent organism previous to sporulation and inclosing the spores, in Sporozoa.
• noun In Sporozoa, the common envelop surrounding the two associated gametocytes.
• noun The resistant covering secreted by many Infusoria previous to going into a resting-stage, or before fission, or for the purpose of resisting draught or other unfavorable conditions of environment.

from the GNU version of the Collaborative International Dictionary of English.

• noun 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.
• noun In old authors, the urinary bladder, or the gall bladder.
• noun (Bot.) One of the bladders or air vessels of certain algæ, as of the great kelp of the Pacific, and common rockweeds (Fuci) of our shores.
• noun 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.
• noun A form assumed by Protozoa in which they become saclike and quiescent. It generally precedes the production of germs. See Encystment.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A pouch or sac without opening, usually membranous and containing morbid matter, which develops in one of the natural cavities or in the substance of an organ.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a small anatomically normal sac or bladderlike structure (especially one containing fluid)
• noun a closed sac that develops abnormally in some body structure