pit viper

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. 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.

Century Dictionary and Cyclopedia

1. n. A venomous serpent of the family Crotalidæ, as a rattlesnake; a pit-headed viper: so called from the characteristic pit between the eyes and the nose. See Bothrophera.

Wiktionary

1. n. Any of various venomous snakes, of the family Crotalinae, found in Asia and the Americas, with a deep pit or fossa on either side of the head that enables them to detect prey by infrared.

GNU Webster's 1913

1. n. (Zoöl.) any viperine snake having a deep pit on each side of the snout. The rattlesnake and copperhead are examples.

WordNet 3.0

1. n. New World vipers with hollow fangs and a heat-sensitive pit on each side of the head