from The American Heritage® Dictionary of the English Language, 4th Edition
- n. The third full moon in a three-month calendrical season that has four full moons.
- n. The second of two full moons occurring in the same month.
- n. Informal A relatively long period of time: I haven't seen you in a blue moon.
from Wiktionary, Creative Commons Attribution/Share-Alike License
- n. The moon tinted towards blue as it appears in the sky, caused by dust or smoke in the atmosphere.
- n. Something absurd.
- n. The third full moon in a quarter that contains four rather than the usual three full moons.
- n. (by extension) A long time.
- n. A flavor of sweet ice cream that is popular in the Midwestern U.S. and is a component of Superman ice cream.
- n. The second of two full moons that occur in the same calendar month.
from the GNU version of the Collaborative International Dictionary of English
- The second full moon occurring in the same month; -- derived from the expression once in a blue moon.
- n. see blue moon in the vocabulary.
from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.
- n. a long time
Sense 2, probably from the rare occurrence whereby the moon appears blue from high amounts of dust in the atmosphere, as from a volcanic eruption.(American Heritage® Dictionary of the English Language, Fourth Edition)
Mistake in Sky & Telescope magazine in 1946, retracted nearly 60 years later. (Wiktionary)