a. The third full moon in a three-month calendrical season that has four full moons.
b. The second of two full moons occurring in the same month.
2. Informal A relatively long period of time: I haven't seen you in a blue moon.
[Sense 2, probably from the rare occurrence whereby the moon appears blue from high amounts of dust in the atmosphere, as from a volcanic eruption.]
The American Heritage® Dictionary of the English Language, Fifth Edition copyright ©2020 by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Indo-European & Semitic Roots Appendices
Thousands of entries in the dictionary include etymologies that trace their origins back to reconstructed proto-languages. You can obtain more information about these forms in our online appendices:
The Indo-European appendix covers nearly half of the Indo-European roots that have left their mark on English words. A more complete treatment of Indo-European roots and the English words derived from them is available in our Dictionary of Indo-European Roots.