Noun

• S: (n) alveolus, air sac, air cell (a tiny sac for holding air in the lungs; formed by the terminal dilation of tiny air passageways)
• S: (n) air sac (any of the thin-walled extensions of the tracheae of insects)
• S: (n) air sac (any of the membranous air-filled extensions of the lungs of birds)
• direct hypernym / inherited hypernym / sister term
• S: (n) sac (a structure resembling a bag in an animal)
• S: (n) cavity, bodily cavity, cavum ((anatomy) a natural hollow or sinus within the body)
• S: (n) structure, anatomical structure, complex body part, bodily structure, body structure (a particular complex anatomical part of a living thing and its construction and arrangement) "he has good bone structure"
• S: (n) body part (any part of an organism such as an organ or extremity)
• S: (n) part, piece (a portion of a natural object) "they analyzed the river into three parts"; "he needed a piece of granite"
• S: (n) thing (a separate and self-contained entity)
• S: (n) physical entity (an entity that has physical existence)
• S: (n) entity (that which is perceived or known or inferred to have its own distinct existence (living or nonliving))
• part holonym