Engelsk definition
A family of snakes comprising four subfamilies: Azemiopinae (the mountain viper, the sole member of this subfamily), Viperinae (true vipers), Crotalinae (pit vipers) and Causinae. They are widespread throughout the world, being found in the United States, Central and South America, Europe, Asia and Africa. Their venoms act on the blood (hemotoxic) as compared to the venom of elapids which act on the nervous system (neurotoxic). (Goin, Goin, and Zug, Introduction to Herpetology, 3d ed, pp333-36)