- Tiny air sacs at the end of the bronchioles in the lungs.
National Cancer Institute
- A small cavity or pit: as a) a socket in the jaw for a tooth; b) any of the small thin-walled air-containing compartments of the lung that are typically arranged in saclike clusters into which an alveolar duct terminates and from which respiratory gases are exchanged with the pulmonary capillaries; c) an acinus of a compound gland; d) any of the pits in the wall of the stomach into which the glands open.
Definition from: Merriam-Webster's Medical Dictionary by Merriam-Webster Inc.
See also Understanding Medical Terminology.