The Collaborative International Dictionary of English v.0.48:

Peacock \Pea"cock`\ (p[=e]"k[o^]k`), n. [OE. pecok. Pea- in this
   word is from AS. pe['a], p[=a]wa, peacock, fr. L. pavo, prob.
   of Oriental origin; cf. Gr. taw`s, taw^s, Per. t[=a]us,
   t[=a]wus, Ar. t[=a]w[=u]s. See Cock the bird.]
   1. (Zool.) The male of any pheasant of the genus Pavo, of
      which at least two species are known, native of Southern
      Asia and the East Indies.
      [1913 Webster]

   Note: The upper tail coverts, which are long and capable of
         erection, are each marked with a black spot bordered by
         concentric bands of brilliant blue, green, and golden
         colors. The common domesticated species is Pavo
         cristatus. The Javan peacock (Pavo muticus) is more
         brilliantly colored than the common species.
         [1913 Webster]

   2. In common usage, the species in general or collectively; a
      peafowl.
      [1913 Webster]

   Peacock butterfly (Zool.), a handsome European butterfly
      (Hamadryas Io) having ocelli like those of peacock.

   Peacock fish (Zool.), the European blue-striped wrasse
      (Labrus variegatus); -- so called on account of its
      brilliant colors. Called also cook wrasse and cook.

   Peacock pheasant (Zool.), any one of several species of
      handsome Asiatic pheasants of the genus Polyplectron.
      They resemble the peacock in color.
      [1913 Webster]

WordNet (r) 3.0 (2006):

peacock butterfly
    n 1: European butterfly having reddish-brown wings each marked
         with a purple eyespot [syn: peacock, peacock butterfly,
         Inachis io]