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