The Collaborative International Dictionary of English v.0.48:

Pilot lamp \Pilot lamp\ or Pilot light \Pilot light\ (Elec.)
   1. 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.]

   2. a small light on an electrical device to indicate whether
      the device is in operation or that the power is on.
      [PJC]

WordNet (r) 3.0 (2006):

pilot lamp
    n 1: indicator consisting of a light to indicate whether power
         is on or a motor is in operation [syn: pilot light,
         pilot lamp, indicator lamp]