pilot lamp

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

• noun A small electric lamp used to indicate that an electric circuit is energized.

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

• (Elec.) A small incandescent telltale lamp on a dynamo or battery circuit to show approximately by its brightness the voltage of the current.
• a small light on an electrical device to indicate whether the device is in operation or that the power is on.

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