1. spherical swelling: a closed, usually spherical, membranous sac that develops in human or other animal tissue and contains fluid or semisolid material. Some types of cysts form when glands are blocked, and most cysts are benign.
2. anatomy hollow organ or cavity: a thin-walled bladder, sac, or vesicle in an animal
3. botany fungi resting spore: in some algae and fungi, a spore that is not undergoing cell division
4. botany air-filled cavity in seaweeds: a small air-filled cavity resembling a bladder that occurs in some seaweeds such as the bladder wrack
5. zoology protective sac enclosing organism: a sac or capsule that encloses and protects some organisms in a dormant or larval stage
6. zoology protective covering around parasite: a protective covering around a parasite, produced by a host or by the parasite itself
[Early 18th century. Via late Latin< Greek kustis "bladder, cyst"]
