blue moon

[ bloo moon ]

noun

1. 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.
2. (formerly) the third full moon in a season with four full moons instead of the usual three.
3. a long, indeterminate stretch of time, as between occurrences of a rare event or phenomenon.
4. 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

1. a very long period of time:

   Such a chance comes once in a blue moon.

blue moon

noun

1. the second full moon occurring within a calendar month
2. once in a blue moon informal.
   very rarely; almost never

Word History and Origins

Origin of blue moon¹

First recorded in 1815–25