Definition of "blue moon" []

• The third full moon in a three-month calendrical season that has four full moons. (noun)
• The second of two full moons occurring in the same month. (noun)
• Informal A relatively long period of time: I haven't seen you in a blue moon. (noun)

American Heritage(R) Dictionary of the English Language, Fifth Edition. Copyright (c) 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.