The Collaborative International Dictionary of English v.0.48:

Vanessa \Van*es"sa\ (v[a^]n*[e^]s"s[.a]), n. [Probably from
   Swift's poem of Cadenus and Vanessa. See Vanessa, in the
   Dictionary of Noted Names in Fiction.] (Zool.)
   Any one of numerous species of handsomely colored butterflies
   belonging to Vanessa and allied genera. Many of these
   species have the edges of the wings irregularly scalloped.
   [1913 Webster]

WordNet (r) 3.0 (2006):

Vanessa
    n 1: painted beauty and red admiral [syn: Vanessa, genus
         Vanessa]