A small incandescent telltale lamp on a dynamo or battery circuit to show approximately by its brightness the voltage of the current. [Webster 1913 Suppl.]
a small light on an electrical device to indicate whether the device is in operation or that the power is on. [PJC]
pilot light, n.
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. [PJC]