Retrospectively, I think most people generally define a "classic" album as one which seems to be a musical snapshot of culture at the time. Which suggests some people might call some albums "classic" even if they lack staying power. Some might argue records by Def Lepard were the perfect embodiment of the 80s mainstream rock sound, but you could also argue their discography is dated and lacks staying power as a result of some of the pillars of the genre in that decade.

That being said, I also use the term "classic" in a more personal, individual sense based on how an album resonates with me emotionally.