The Collaborative International Dictionary of English v.0.48:

Danaus \Danaus\ n.
   the type genus of the Danaidae, including the monarch
   butterfly, Danaus plexippus.

   Syn: genus Danaus.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

Danaus plexippus
    n 1: large migratory American butterfly having deep orange wings
         with black and white markings; the larvae feed on milkweed
         [syn: monarch, monarch butterfly, milkweed butterfly,
         Danaus plexippus]