The Collaborative International Dictionary of English v.0.48:

Monarch \Mon"arch\, n. [F. monarque, L. monarcha, fr. Gr. ?, ?;
   mo`nos alone + ? to be first, rule, govern. See Archi-.]
   1. A sole or supreme ruler; a sovereign; the highest ruler;
      an emperor, king, queen, prince, or chief.
      [1913 Webster]

            He who reigns
            Monarch in heaven, . . . upheld by old repute.
                                                  --Milton.
      [1913 Webster]

   2. One superior to all others of the same kind; as, an oak is
      called the monarch of the forest.
      [1913 Webster]

   3. A patron deity or presiding genius.
      [1913 Webster]

            Come, thou, monarch of the vine,
            Plumpy Bacchus.                       --Shak.
      [1913 Webster]

   4. (Zool.) A very large red and black butterfly (Danais
      Plexippus); -- called also milkweed butterfly and
      monarch butterfly.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

monarch butterfly \mon"arch but"ter*fly\, n. (Zool.)
   A very large red and black butterfly (Danais Plexippus)
   having striking orange-brown wings with black veins in a
   reticulated pattern; -- called also milkweed butterfly and
   monarch. Its larvae feed on the leaves of the milkweed.
   [1913 Webster +PJC]

WordNet (r) 3.0 (2006):

milkweed butterfly
    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]