- 1: A membranous and usually fluid-filled pouch (as a cyst, vacuole, or cell) in a plant or animal.
2: A small abnormal elevation of the outer layer of skin enclosing a watery liquid: blister.
3: A pocket of embryonic tissue that is the beginning of an organ.
Definition from: Merriam-Webster's Medical Dictionary by Merriam-Webster Inc.
See also Understanding Medical Terminology.