An acinus (/ˈæsɨnɨs/) (plural, acini; adjective, acinar [/ˈæsɨnər/] or acinous) refers to any cluster of cells that resembles a many-lobed berry, such as a raspberry (acinus is Latin for "berry"). The berry-shaped termination of an exocrine gland, where the secretion is produced, is acinar in form, as is the alveolar sac containing multiple alveoli in the lungs.
The thyroidfollicles can also be considered of acinar formation but in this case the follicles, being part of an endocrine gland, act as a hormonal deposit rather than to facilitate secretion.
Mucous acini usually stain pale, while serous acini usually stain dark.
The term "acinus" is considered synonymous with alveolus by some sources, but not all.