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