The heat-sensing facial pit found in pit vipers is called the loreal pit.
A loreal pit is a specialized infrared-sensing organ located between the eye and nostril on each side of a pit viper’s head. It detects thermal radiation from warm objects, including prey, and helps the snake build a temperature-based view of its surroundings.
Pit vipers include rattlesnakes, copperheads, cottonmouths, and many Asian and American vipers. Their infrared system can assist hunting in darkness, when ordinary vision is less useful. The pits do not produce heat; they detect differences in infrared radiation.
The name comes from the loreal region of the head, near the lore scale. This feature distinguishes pit vipers from many other venomous snakes, including true cobras and mambas, which do not possess loreal pits.