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