- A sac or capsule filled with fluid.
National Cancer Institute
- 1: A closed sac having a distinct membrane and developing abnormally in a body cavity or structure.
2: A body resembling a cyst: as a) a capsule formed about a minute organism going into a resting or spore stage; also: this capsule with its contents b) a resistant cover about a parasite produced by the parasite or the host.
Definition from: Merriam-Webster's Medical Dictionary by Merriam-Webster Inc.
See also Understanding Medical Terminology.