white whale

Short definition from Wiktionary

Noun

1. 1
   Synonym of beluga (“the beluga whale (Delphinapterus leucas), a cetacean found in the Arctic Ocean”)
2. 2
   Any whale of the family Monodontidae, which comprises the beluga whales and narwhals (Monodon monoceros), and some extinct species.
3. 3
   An obsession, a monomania.
4. 4
   A printing plate, used to manufacture a particular sports card, that is then issued as a collectible itself.