Butterfly But"ter*fly`, n.; pl. Butterflies. [Perh. from the color of a yellow species. AS. buter-fl[=e]ge, buttor-fle['o]ge; cf. G. butterfliege, D. botervlieg. See Butter, and Fly.] (Zo["o]l.) A general name for the numerous species of diurnal Lepidoptera. Note: [See Illust. under Aphrodite.] Asclepias butterfly. See under Asclepias. Butterfly fish (Zo["o]l.), the ocellated blenny (Blennius ocellaris) of Europe. See Blenny. The term is also applied to the flying gurnard. Butterfly shell (Zo["o]l.), a shell of the genus Voluta. Butterfly valve (Mech.), a kind of double clack valve, consisting of two semicircular clappers or wings hinged to a cross rib in the pump bucket. When open it somewhat resembles a butterfly in shape.