The Collaborative International Dictionary of English v.0.48:

lightbulb \lightbulb\, light bulb \light bulb\n.
   A glass bulb with metallic contacts on the outside connected
   to a wire filament (usually tungsten) inside that emits light
   when heated by passage of electricity through the filament;
   -- used as the replaceable light-emitting component of an
   electric lamp, flashlight, lighting fixture, etc.

   Syn: light bulb, bulb, incandescent lamp, electric light,
        electric-light bulb.
        [WordNet 1.5 +PJC]

WordNet (r) 3.0 (2006):

lightbulb
    n 1: electric lamp consisting of a transparent or translucent
         glass housing containing a wire filament (usually tungsten)
         that emits light when heated by electricity [syn: light
         bulb, lightbulb, bulb, incandescent lamp, electric
         light, electric-light bulb]