noun

1. any of certain saclike structures in plants or animals
2. such a structure or pocket in the body when abnormal and filled with fluid or diseased matter
3. a spherical, usually thick membrane, resistant to freezing, drying, etc., with which certain organisms are surrounded when in a resting stage

Etymology: ModL cystis < Gr kystis, sac, bladder < ? IE base *us-, var. of *wes-, to wheeze, blow