cyst Definition

cyst
noun

1. pathol.
An abnormal sac that contains fluid, semi-solid material or gas.
2. anatomy.
Any normal sac or closed cavity.
3. biol.
A tough outer membrane which surrounds certain organisms, eg bacteria, protozoa, etc, during the resting stage in their life cycle, thus protecting them from unfavourable environmental conditions.

Etymology: 18c: from Greek kystis bladder or pouch.