Definition of "blue moon"

(countable) An extra full moon that appears in a subdivision of a year, namely:

1. The third full moon in a season that contains four rather than the usual three full moons.  examples

2. The second of two full moons that occur in the same calendar month (a definition created by mistake in the 20th century).  examples

(countable, rare) The moon tinted towards blue as it appears in the sky, caused by dust or smoke in the atmosphere.

(countable, idiomatic, by extension) A long time.  examples

(uncountable) A bright blue, sweet-flavored ice cream that is popular in the Midwestern U.S. and is a component of Superman ice cream.  examples