A fantastic, impracticable plan or desire
(Noun)
Synonyms:
  1. castle in the air
  2. chimera
  3. dream
  4. fantasy
  5. bubble
  6. airy hope
  7. castle in the sky
  8. daydream
  9. false hope
  10. illusion
  11. fool-s-paradise
  12. golden dream
  13. rainbow
  14. pie-in-the-sky
  15. unrealistic hope
  16. wishful-thinking