Definition of ALVEOLUS. 1. : a small cavity or pit: as a: a socket in the jaw for a tooth b: a small air-containing compartment of the lungs in which the bronchioles terminate and from which respiratory gases are exchanged with the pulmonary capillaries c: an acinus of a compound gland d: a cell of a honeycomb.2.. : a small cavity or pit: as a: a socket in the jaw for a tooth b: a small air-containing compartment of the lungs in which the bronchioles terminate and from which respiratory gases are exchanged with the pulmonary capillaries c: an acinus of a compound gland d: a cell of a honeycomb. 2.