viper, any of a large number of heavy-bodied, poisonous snakes of the family Viperidae, characterized by erectile, hypodermic fangs. The fangs are folded back against the roof of the mouth except when the snake strikes. Vipers are distributed throughout Eurasia and Africa. They range in size from under 1 ft (30 cm) to nearly 6 ft (2 m) and often have zigzag or diamond patterns. Best known is the common European viper, or adder (Vipera berus), distributed throughout Europe and N Asia. It feeds on small mammals and lizards and hibernates in winter. Its venom is rarely fatal to humans. The asp viper, or asp (V. aspis), is a smaller species inhabiting S Europe. The greatest variety of vipers is found in Africa. The brightly patterned Gaboon viper, Bitis gabonica, is the longest of the vipers and has a body diameter of up to 6 in. (15 cm). The puff adder, B. arietans, found over most of Africa and in Arabia, is a highly venomous species whose bite is often fatal. The pit vipers of the Americas, including the rattlesnake, copperhead, water moccasin, fer-de-lance, and bushmaster, are classified in a separate family. Vipers are classified in the phylum Chordata, subphylum Vertebrata, class Reptilia, order Squamata, family Viperidae.

pit viper, poisonous snake of the family Crotalidae, primarily a New World family. Like the Old World true vipers (family Viperidae), pit vipers have long, hollow, erectile fangs that are folded back against the roof of the mouth except when the snake is striking. In addition, the pit vipers have developed special organs of heat reception that help them to sense warm-blooded animals, an ability that is especially useful at night, when many of them hunt. These organs consist of pits, for which the group is named, located just behind the nostrils and covered with a temperature-sensitive membrane. Some pit vipers may also use these organs to find cool refuges from inhospitable daytime temperatures.

The largest group of pit vipers is the rattlesnake genus Crotalus, found in North, Central, and N South America. Other New World forms are the fer-de-lance (genus Bothrops) and the bushmaster (genus Lachesis). The genus Ancistrodon includes the copperhead and water moccasin, as well as about a dozen Asian species. Pit vipers are classified in the phylum Chordata, subphylum Vertebrata, class Reptilia, family Crotalidae.

Any of about 200 species (family Viperidae) of venomous snakes in two subfamilies: Viperinae (Old World vipers of Europe, Asia, and Africa) and Crotalinae (pit vipers). Two long, hollow, venom-injecting fangs attached to the movable bones of the viper's upper jaw can be folded back in the mouth when not in use. Vipers range in length from less than 12 in. (30 cm) to more than 10 ft (3 m). They eat small animals and hunt by striking, then trailing, their prey. Many Old World vipers are terrestrial; a few are arboreal or burrowers. Most bear live young.

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