cyst 
(sĭst)
(sĭst)n.
1. An abnormal membranous sac in the body containing a gaseous, liquid, or semisolid substance.
2. A sac or vesicle in the body.
3. Biology A small capsulelike sac that encloses certain organisms in their dormant or larval stage.
4. Botany A thick-walled resting spore, as in certain algae or fungi.
[New Latin cystis, from Greek kustis, bladder; see kwes- in the Appendix of Indo-European roots.]