A hallucination or crazy idea that someone who's generally drunk or high comes up with, then continues to believe during the hangover and usually, after they sober up.
Donny: MAN, I saw like, kids on the moon last night. They were up there, in, like, spacesuits, and I could SEE them. They must have been like giants or something!
Jim: Dude, you had a missing unicorn. GET OVER IT.