The Collaborative International Dictionary of English v.0.48:

Cyst \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.]
          [1913 Webster]

   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.
      [1913 Webster]

   3. (Zool.)
      (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.
          [1913 Webster]

WordNet (r) 3.0 (2006):

cyst
    n 1: a closed sac that develops abnormally in some body
         structure
    2: a small anatomically normal sac or bladderlike structure
       (especially one containing fluid) [syn: vesicle, cyst]

Moby Thesaurus II by Grady Ward, 1.0:

49 Moby Thesaurus words for "cyst":
   benign tumor, boil, bump, bunion, callosity, callus, cancer,
   carbuncle, carcinoma, corn, dilatation, dilation, distension,
   edema, excrescence, fungosity, fungus, furuncle, growth,
   intumescence, lump, malignant growth, metastatic tumor, mole,
   morbid growth, neoplasm, nevus, nonmalignant tumor, outgrowth,
   pimple, pock, proud flesh, pustule, rising, sarcoma,
   sebaceous cyst, swell, swelling, swollenness, tumefaction,
   tumescence, tumidity, tumor, turgescence, turgescency, turgidity,
   verruca, wart, wen