blue moon

noun6

1. •
   An extra full moon that appears in a subdivision of a year.
2. •
   A long time.
3. •
   The third full moon in a season that contains four rather than the usual three full moons.
4. •
   The second of two full moons that occur in the same calendar month (a definition created by mistake in the 20th century).
5. •
   (rare) The moon tinted towards blue as it appears in the sky, caused by dust or smoke in the atmosphere.
6. •
   A bright blue, sweet-flavored ice cream that is popular in the Midwestern U.S. and is a component of Superman ice cream.