any of a subfamily (Crotalinae) of poisonous vipers with a prominent, heat-sensitive pit on each side of the head, as the rattlesnake or copperhead
See pit viper in American Heritage Dictionary 4
noun
Any of various venomous snakes of the family Crotalidae, such as a copperhead, rattlesnake, or fer-de-lance, characterized by a small sensory pit below each eye.