white whale

noun4

1. •
   Any whale of the family Monodontidae, which comprises the beluga whales and narwhals (Monodon monoceros), and some extinct species.
2. •
   (figurative) An obsession, a monomania.

   • No … no sapphires. It looks like Lester Velasquez was chasing his white whale. There never were any sapphires inside that diner. — 2008
3. •
   Synonym of beluga (“the beluga whale, Delphinapterus leucas, a cetacean found in the Arctic Ocean”)

   • The English called the beluga the "white whale", a name often still used (and not to be confused with the white whale of Moby-Dick, which was an albino sperm whale). — 2007
4. •
   (figurative) A printing plate, used to manufacture a particular sports card, that is then issued as a collectible itself.