pit viper

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Any of various venomous snakes of the subfamily Crotalinae, such as the copperhead, rattlesnake, or fer-de-lance, characterized by a small sensory pit below each eye.

from The Century Dictionary.

• noun 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.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun 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.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun New World vipers with hollow fangs and a heat-sensitive pit on each side of the head