\Cyst\ (s[i^]st), n. [Gr. ky`stis bladder, bag, pouch, fr.
ky`ein to be pregnant. Cf. {Cyme}.]
1. (Med.)
   (a) 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.
   (b) In old authors, the urinary bladder, or the gall
       bladder. [Written also {cystis}.]
2. (Bot.) One of the bladders or air vessels of certain
   alg[ae], as of the great kelp of the Pacific, and common
   rockweeds ({Fuci}) of our shores. --D. C. Eaton.
3. (Zo["o]l.)
   (a) 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.
   (b) A form assumed by Protozoa in which they become
       saclike and quiescent. It generally precedes the
       production of germs. See {Encystment}.