The Collaborative International Dictionary of English v.0.48:

moccasin \moc"ca*sin\, n. [An Indian word. Algonquin makisin.]
   [Sometimes written moccason.]
   1. A shoe made of deerskin, or other soft leather, the sole
      and upper part being one piece. It is the customary shoe
      worn by the American Indians.
      [1913 Webster]

   2. (Zool.) A poisonous snake of the Southern United States.
      The water moccasin (Ancistrodon piscivorus syn.
      Agkistrodon piscivorus, also called cottonmouth and
      cottonmouth water moccasin) is usually found in or near
      water. Above, it is olive brown, barred with black;
      beneath, it is brownish yellow, mottled with darker. The
      upland moccasin is Ancistrodon atrofuscus. They resemble
      rattlesnakes, but are without rattles.
      [1913 Webster]

   Moccasin flower (Bot.), a species of lady's slipper
      (Cypripedium acaule) found in North America. The lower
      petal is two inches long, and forms a rose-colored
      moccasin-shaped pouch. It grows in rich woods under
      coniferous trees.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

cottonmouth \cottonmouth\ n.
   venomous semiaquatic snake (Agkistrodon piscivorus) of
   swamps in southern U.S.; -- called also water moccasin,
   cottonmouth moccasin, and cottonmouth water moccasin. It
   grows to a length of about 4 feet. The name refers to the
   whiteness of the lips and inside of the mouth.

   Syn: water moccasin, cottonmouth moccasin, Agkistrodon
        piscivorus.
        [WordNet 1.5 +PJC]

WordNet (r) 3.0 (2006):

cottonmouth
    n 1: venomous semiaquatic snake of swamps in southern United
         States [syn: water moccasin, cottonmouth, cottonmouth
         moccasin, Agkistrodon piscivorus]