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