Pulmonary acinus: The ending of a tiny airway in the lung, where the alveoli (air sacs) are located.

In anatomy, an acinus is a round cluster of cells, usually epithelial cells, that looks somewhat like a knobby berry.

The word "acinus" means "berry" in Latin. (The plural is "acini".) There are also acini, round clusters of epithelial cells, in the salivary glands and in the pancreas.