(1) terminal dilation of the alveolar ducts, which give rise to alveoli in the lung; a small air chamber in the pulmonary tissue from which the pulmonary alveoli project like bays and into which an alveolar duct opens; SYN: sacculus alveolaris [TA]. (2), in birds, air-containing extensions of bronchi that connect with bone cavities. SYN: air sac.