cyst
cyst \'sist\ n [NL cystis, fr. Gk kystis bladder, pouch] 1: a closed sac having a distinct membrane and developing abnormally in a c avity or structure of the body 2: a body resembling a cyst : as 2a: a resting spore of many algae 2b: an air vesicle (as of a rockweed) 2c: a capsule formed about a minute organism going into a resting or spore stage; also : this capsule with its contents 2d: a resistant cover about a parasite produced by the parasite or the host