water moccasin

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A semiaquatic pit viper (Agkistrodon piscivorus) of lowlands and swampy regions of the southeast United States.
• noun Any of various similar but harmless water snakes.

from The Century Dictionary.

• noun A water-adder: a name applied with little discrimination in the United States to several species of aquatic snakes; properly, the venomous Toxicophis or Ancistrodon piscivorus, with which the harmless Tropidonotus (or Nerodia) sipedon is sometimes confounded. See water-snake, and cut under moccasin.

from the GNU version of the Collaborative International Dictionary of English.

• (Zoöl.) A venomous North American snake (Ancistrodon piscivorus) allied to the rattlesnake but destitute of a rattle. It lives in or about pools and ponds, and feeds largely of fishes. Called also water snake, water adder, water viper.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun any of numerous North American water snakes inhabiting fresh waters
• noun venomous semiaquatic snake of swamps in southern United States