light bulb

noun3

1. •
   An evacuated glass bulb containing a metal filament which is heated by electrical resistance to produce light.

   • Atticus did something that interested us: he came into the livingroom carrying a long electrical extension cord. There was a light bulb on the end. — 1960
2. •
   An article that resembles such a bulb and converts electricity to light by any process.
3. •
   (figurative) Used in reference to the sudden arrival of a realization, an inspiration, an idea, or the like.

   • Figuratively speaking, a light-bulb flashed over Galvin's head early this year, and in March–along […] — 1949