white whale

plural white whales

1. 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, Nancy Lord, “Prologue”, in Beluga Days: Tracking the Endangered White Whale, pages 22–23

   Synonyms: melonhead, sea canary, beluga

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

   For nearly four years, Snapchat has been Facebook’s white whale.

   2016 August 2, Mike Isaac, “Instagram takes a page from Snapchat, and takes aim at it, too”, in The New York Times, →ISSN, archived from the original on 09 Jan 2021

   2 more examples

   No … no sapphires. It looks like Lester Velasquez was chasing his white whale. There never were any sapphires inside that diner.

   2008, Glen Inglis, Broken Heroes, page 228

   After being lofted into space by a Falcon Super Heavy, his next rocket, the Starship, will light out for the moon, land there, take off and return to Earth, with no stages expended on the lunar journey. This so-called single-stage-to-orbit model has been the white whale of rocket designers for generations.

   2021 December 13, Molly Ball, Jeffrey Kluger, Alejandro de la Garza, “Elon Musk: Person of the Year 2021”, in TIME, archived from the original on 13 Dec 2021

4. figurativelycollectible card gamesA printing plate, used to manufacture a particular sports card, that is then issued as a collectible itself.