water moccasin

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. A semiaquatic pit viper (Agkistrodon piscivorus) of lowlands and swampy regions of the southern United States. Also called cottonmouth.
2. n. Any of various similar but harmless water snakes.

Century Dictionary and Cyclopedia

1. n. 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.

GNU Webster's 1913

1. (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.

WordNet 3.0

1. n. any of numerous North American water snakes inhabiting fresh waters
2. n. venomous semiaquatic snake of swamps in southern United States