Source : Webster's Revised Unabridged Dictionary (1913)

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.

Source : WordNet®

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