lightbulb

noun

1. 1.An evacuated glass bulb containing a metal filament which is heated by electrical resistance to produce light.
2. 2.(by extension) 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.An uninvited third person joining a couple's date who prevents or hinders romantic interactions.