Medical Definition of viper
1: a common Eurasian venomous snake of the genus Vipera (V. berus) that attains a length of about two feet (0.6 meter), varies in color from red, brown, or gray with dark markings to black, and whose bite is usually not fatal to humans; broadly : any snake of an Old World subfamily (Viperinae) of the family Viperidae
2: pit viper
3: a venomous or reputedly venomous snake
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