Coat | definition of coat by Medical dictionary dictionary, online dictionary, dictionaries, encyclopedia, thesaurus, medical dictionary, English dictionary, definition, legal dictionary, computer dictionary, law, dictionary, free, financial glossa coat [kōt] 1. a membrane or other structure covering or lining a part or organ; in anatomic nomenclature called tunica. 2. the layer or layers of protective protein surrounding the nucleic acid in a virus. See also capsid. buffy coat the thin yellowish layer of leukocytes overlying the packed erythrocytes in centrifuged blood. coat (kōt), 1. The outer ...