light bulb

Short definition from Wiktionary

Noun

1. 1
   An evacuated glass bulb containing a metal filament which is heated by electrical resistance to produce light.
2. 2
   An article that resembles such a bulb and converts electricity to light by any process.
3. 3
   Used in reference to the sudden arrival of a realization, an inspiration, an idea, or the like.
4. 4
   Alternative form of light bulb.