lightbulb

1. An evacuated glass bulb containing a metal filament which is heated by electrical resistance to produce light.
2. (by extension) An article that resembles such a bulb and converts electricity to light by any process.

   a fluorescent light bulb

3. Used in reference to the sudden arrival of a realization, an inspiration, an idea, or the like.

   a light bulb moment

noun

An uninvited third person joining a couple's date who prevents or hinders romantic interactions.