will o' the wisp

noun3

1. •
   Any of several kinds of pale, flickering light, appearing over marshland in many parts of the world with diverse folkloric explanations and multiple possible scientific explanations including bioluminescence and chemiluminescence.
2. •
   A flickering light or lantern, especially over bogs, swamps or marshes, held by a supernatural creature who uses it to lure travelers at night to their demise.

   • Will o' the wisps / Misguiding your path / You can't throw a curse / Without takin' their wrath — 1988
3. •
   (figurative) A delusionary or otherwise unachievable goal that one feels compelled to pursue.

   • I happen to think the brothers’ pi project is a will-o’-the-wisp, and is one of the least interesting things they’ve ever done. But what do I know? — 1992