acinus Syllabification: (ac·i·nus)Pronunciation: /ˈasənəs/Definition of acinus noun (plural acini /-ˌnī/) Anatomy 1a small saclike cavity in a gland, surrounded by secretory cells. 2a region of the lung supplied with air from one of the terminal bronchioles. Derivatives acinar adjective Origin: mid 18th century: Latin, literally 'a kernel'acinus in other Oxford dictionaries Definition of acinus in the British & World English dictionary