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a. The third full moon in a three-month calendrical season that has four full moons.
b. The second of two full moons occurring in the same month.
2. Informal A relatively long period of time: I haven't seen you in a blue moon.
[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.]
1. the second full moon occurring within a calendar month
2. once in a blue moon informal very rarely; almost never
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|Noun||1.||blue moon - a long time; "something that happens once in blue moon almost never happens"|
colloquialism - a colloquial expression; characteristic of spoken or written communication that seeks to imitate informal speech