noun, plural:

(general) A small case, envelope, or covering.

(1) A membranous sac or integument, especially surrounding partly or wholly a bodily structure or organ.
(2) Any of the two strata of white matter in the cerebrum.

(1) The sporangium of ferns, mosses, algae and fungi.
(2) A type of dehiscent fruit made up of two or more carpels (e.g. poppy).

(microbiology) The mucopolysaccharide layer that lies outside the cell wall of bacteria.

(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 anatomy, a capsule refers to the membranous sheath that surrounds a bodily organ such as kidney, or the fibrous tissues that surrounds a joint.

In botany, a capsule is a dehiscent fruit which, at maturity, split apart (dehisce) to release the seeds within.

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)

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