- 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.
- (formerly) the third full moon in a season with four full moons instead of the usual three.
- a long, indeterminate stretch of time, as between occurrences of a rare event or phenomenon.
- 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.
- a very long period of time: Such a chance comes once in a blue moon.
- the second full moon occurring within a calendar month
- very rarely; almost never
