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]

The Collaborative International Dictionary of English v.0.48:

Pilot light \Pi"lot 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]

WordNet (r) 3.0 (2006):

pilot light
    n 1: small auxiliary gas burner that provides a flame to ignite
         a larger gas burner [syn: pilot burner, pilot light,
         pilot]
    2: indicator consisting of a light to indicate whether power is
       on or a motor is in operation [syn: pilot light, pilot
       lamp, indicator lamp]