Definition of blue moon in English:

blue moon


  • 1A phenomenon whereby the moon appears bluish owing to smoke or dust particles in the atmosphere.

    ‘people saw blue moons for years after the 1883 eruption of Krakatoa’
  • 2A second full moon in a calendar month.

    ‘tomorrow's full moon is a blue moon’
    as modifier ‘it can be no coincidence that July is a blue moon month’


  • once in a blue moon

    • informal Very rarely.

      ‘he comes round once in a blue moon’
      hardly ever, almost never, scarcely ever, rarely, very seldom
      View synonyms