A "blue moon" is a term used to describe a rare occurrence of either the second full moon in a single calendar month or the third full moon in a season that has four full moons. It does not usually mean the moon is actually blue in color. The phrase has become a metaphor for something that happens rarely, hence the saying "once in a blue moon." The modern popular definition of a blue moon is a second full moon occurring within the same calendar month, which happens roughly every two to three years. The original and less known definition, used in the past according to the Maine Farmers' Almanac, refers to the third full moon in a season with four full moons. The moon can actually appear blue on very rare occasions, but this is due to atmospheric conditions such as dust or smoke particles from large volcanic eruptions that scatter red light and allow blue light to pass through. However, these rare blue appearances of the moon are unrelated to the calendrical blue moon phenomenon. In summary:

• The commonly accepted "blue moon" is the second full moon in a month, a relatively rare event.
• Another definition is the third full moon in a season with four full moons.
• Actual blue-colored moons are rare and caused by atmospheric effects, not the calendar.

This term originated from a 1940s misunderstanding but has since become an established part of popular vernacular to describe rare lunar events.