Source : Webster's Revised Unabridged Dictionary (1913)

Pilot lamp \Pilot lamp\ or light \light\ . (Elec.)
   A small incandescent telltale lamp on a dynamo or battery
   circuit to show approximately by its brightness the voltage
   of the current.

Source : WordNet®

pilot lamp
     n : indicator consisting of a light to indicate whether power is
         on or a motor is in operation [syn: {pilot light}, {indicator
         lamp}]