blue moon

Short definition from Wiktionary

Noun

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