In birds, a thin-walled extension of the lungs. There are nine of these and they extend into the abdomen and bones, effectively increasing lung capacity. In mammals, it is another name for the alveoli in the lungs, and in some insects, for widenings of the trachea.
1. Any of the membranous air-filled extensions of the lungs of birds.
2. Any of the thin-walled extensions of the tracheae of insects.
3. A tiny sac for holding air in the lungs; formed by the terminal dilation of tiny air passageways