blue moon
Americannoun
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a second full moon in a month, occurring every two to three years as a result of the discrepancy between a lunar year and a solar year.
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(formerly) the third full moon in a season with four full moons instead of the usual three.
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a long, indeterminate stretch of time, as between occurrences of a rare event or phenomenon.
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an uncommon phenomenon in which the moon appears to have a blue tinge because of the filtering effect of smoke or dust particles in the atmosphere.
idiom
noun
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the second full moon occurring within a calendar month
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informal very rarely; almost never
Other Word Forms
Noun Inflected Forms
Etymology
Origin of blue moon
First recorded in 1815–25
Example Sentences
Examples are provided to illustrate real-world usage of words in context. Any opinions expressed do not reflect the views of Dictionary.com.
Once in a blue moon, a person is willing to do the hard internal work that leads to real, sustainable change.
From Salon ● Mar. 21, 2025
Monday’s super blue moon will kick off four consecutive supermoons this year.
From Los Angeles Times ● Aug. 19, 2024
Monday's full supermoon - also a blue moon - turned red in a dramatic way on Sunday night.
From BBC ● Aug. 19, 2024
It’s considered both a supermoon and a blue moon because of its proximity to earth and its timing during the season.
From Los Angeles Times ● Aug. 19, 2024
With the flour and oil she got from trading eggs, Miss Maggie baked bread that made me happier than if I’d had cake, which I tasted only once in a blue moon.
From "Beyond the Bright Sea" by Lauren Wolk
Definitions and idiom definitions from Dictionary.com Unabridged, based on the Random House Unabridged Dictionary, © Random House, Inc. 2023
Idioms from The American Heritage® Idioms Dictionary copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.
