pilot light

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

• noun A small jet of gas that is kept burning in order to ignite a gas burner, as in a stove.

from The Century Dictionary.

• noun A very small gaslight kept burning beside a large burner, so that when the flow through the main burner is turned on it will be automatically lighted by the pilot-light. It is usually protected by a shield from being accidentally blown out. See by-pass.
• noun A lamp placed on a dynamo to show that it is working properly.
• noun A small burner burning continuously, used in motor-car practice to ignite the main burner when the demand for power exceeds the supply of energy stored in the generator from a previous ignition.

from the GNU version of the Collaborative International Dictionary of English.

• noun a small gas flame kept burning continuously on a stove, water heater, or other gas-burning device, so as to allow immediate ignition of the main flame when the gas flow is turned on.

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

• noun A small, persistent flame used to ignite gas.

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

• noun indicator consisting of a light to indicate whether power is on or a motor is in operation
• noun small auxiliary gas burner that provides a flame to ignite a larger gas burner