(general) A small case, envelope, or covering.
(pharmacology) A small soluble container enclosing a dose of an oral medicine or vitamin.
(pathology) An encapsulated material, especially one that results from a cellular or immune response to a foreign body.
In microbiology, the capsule help protect bacteria from phagocytosis as well as from desiccation. It also helps them to adhere to surfaces and cells. Hence, it is considered a virulence factor. It is found most commonly among gram-negative bacteria, such as Escherichia coli.
Word origin: French, from Latin capsula, diminutive of capsa, box.
Related forms: capsular (adjective)