What does "butterfly" mean?

Butterfly: A general name for the numerous species of diurnal Lepidoptera. Note: [See Illust. under Aphrodite.] Asclepias butterfly. See under Asclepias. -- Butterfly fish (Zoöl.), the ocellated blenny (Blennius ocellaris) of Europe. See Blenny. The term is also applied to the flying gurnard. -- Butterfly shell (Zoöl.), 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.