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.