A blue moon is an additional full moon that appears in a subdivision of a year, either the third of four full moons in a season or the second full moon in a calendar month. The phrase "blue moon" has nothing to do with the actual color of the moon, although a visually blue moon (the moon appearing with a bluish tinge) may occur under certain atmospheric conditions, such as volcanic eruptions or fires releasing particles in the atmosphere of just the right size to preferentially scatter red light. Blue moons that are indeed the color blue are very rare and have nothing to do with the type of the month or the moons phases, instead they are caused by atmospheric conditions.

There are two different definitions for a blue moon:

• Seasonal Blue Moon: The third full moon of an astronomical season that has four full moons.
• Monthly Blue Moon: The second full moon in a calendar month with two full moons.

The monthly blue moon is nowadays considered the second definition of a blue moon rather than a mistake. The historical origins of the term and its two definitions are shrouded in a bit of mystery and, by many accounts, an interpretation error.