Pilot light
A small standing flame that ignites the burner in some gas appliances.
A pilot light is the small, constant flame in older gas water heaters, furnaces, and stoves that lights the main burner when heat is called for. A [thermocouple](/glossary/thermocouple) senses the flame and shuts off the gas if it goes out. If your pilot won't stay lit, the thermocouple is usually the inexpensive culprit. Many modern appliances use electronic ignition instead and have no standing pilot.