Medical Definition of pit viper
: any of various mostly New World highly specialized venomous snakes (as the rattlesnake, copperhead, water moccasin, and fer-de-lance) that belong to a subfamily (Crotalinae) of the family Viperidae, that have a small depression on each side of the head between the eye and the nostril, and that have hollow perforated fangs usually folded back in the upper part of the mouth but erected in striking
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