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
Tuesday’s moon follows the Aug. 19 super blue moon.
From Los Angeles Times • Sep. 17, 2024
Despite being called a blue moon, the moon doesn't actually turn blue.
From BBC • Aug. 19, 2024
The full moon, which will be both a supermoon and a blue moon, will grace the skies Monday and will appear full until early Wednesday morning, according to NASA.
From Los Angeles Times • Aug. 19, 2024
Once in a blue moon, I’ve seen my mother try something similar, but not often.
From "Catching Fire" by Suzanne Collins
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.
