- a sac containing air.
- alveolus (def. 2).
- any of certain cavities in a bird's body connected with the lungs.
- a saclike dilation of the wall of a trachea in many insects.
- any of the membranous air-filled extensions of the lungs of birds, which increase the efficiency of gaseous exchange in the lungs
- any of the thin-walled extensions of the tracheae of insects having a similar function
