moccasin, (genus Agkistrodon), either of two venomous aquatic New World snakes of the viper family (Viperidae): the water moccasin (Agkistrodon piscivorus) or the Mexican moccasin (A. bilineatus). Both are pit vipers (subfamily Crotalinae), so named because of the characteristic sensory pit between each eye and nostril.
The water moccasin inhabits marshy lowlands of the southeastern United States. It is also known as the cottonmouth, because it threatens with the mouth open, showing the white interior. It measures up to 1.5 metres (5 feet) in length and is brown with darker crossbands or completely black. A dangerous snake with a potentially lethal bite, the cottonmouth tends to stand its ground or move slowly away when alarmed. It eats almost any small animal, including turtles, fish, frogs, and birds.
The Mexican moccasin, or cantil, is found in lowland regions from the Rio Grande to Nicaragua. It is a dangerous snake and is brown or black with narrow irregular whitish bars on its back and sides. It is usually about 1 metre (3.3 feet) long. Moccasins are live-bearers (viviparous) rather than egg layers. For highland moccasin, see copperhead.