a large dark grey venomous snake, Agkistrodon piscivorus, of swamps in the southern US: family Crotalidae (pit vipers)
Also called: cottonmouth
water moccasin in American English
US
a large, poisonous, olive-brown pit viper (Agkistrodon piscivorus) with dark crossbars, found along or in rivers and swamps in the SE U.S.; cottonmouth: often confused with various harmless snakes, esp. several water snakes (genus Natrix)