Definition of "white whale"
(figuratively)
An obsession, a monomania.
Quotations
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 Magazine