The Collaborative International Dictionary of English v.0.48:

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.] (Zool.)
   A general name for the numerous species of diurnal
   Lepidoptera.

   Note: [See Illust. under Aphrodite.]
         [1913 Webster]

   Asclepias butterfly. See under Asclepias.

   Butterfly fish (Zool.), the ocellated blenny (Blennius
      ocellaris) of Europe. See Blenny. The term is also
      applied to the flying gurnard.

   Butterfly shell (Zool.), 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.
      [1913 Webster]

WordNet (r) 3.0 (2006):

butterfly fish
    n 1: small usually brilliantly colored tropical marine fishes
         having narrow deep bodies with large broad fins; found
         worldwide