What's the difference between a parallel and alternate universe? Are these important differences? Can these two terms be interchanged?
For this, you're going to need to consider a few things. First and foremost, what is the core difference? I'd say an alternate universe is a universe similar to the main one, only with one or two key differences. Whereas a parallel universe is one with all the same players, but just different events altogether occurring. Consider the Marvel Comics Universe. The 616 universe is the core reality, and there are numerous alternate and parallel realities. Something like Marvel Zombies, where there is one key event that changes everything, would be an alternate universe. On the other hand, the Ultimate Universe is totally different with completely different components and the like...but has all the core players from the main universe Ergo, parallel universe. Also consider remakes and film reboots. Genre remakes especially. Movies like the Halloween series has a surprisingly fragmented continuity by design. The first two films are core canon, but 4-6 are in an alternate universe to 7-8, with the core difference being Laurie Strode's survival and legacy. Whereas Rob Zombie's remake series is a parallel reality with nothing in common with the core films. And don't even get me started on the films with Batman. Or James Bond, even. Or...oh God...Godzilla. – agramugl8 years ago
I'd say that alternate is recognizably the same universe, while parallel is not. Alternate universes typically have generally the same general set-up as our own world, but differentiated recently in the timeline. I see this the most in DC comics: In "The Flash" CW show, there are alternate Earths with similar versions of each character (the same happen in Doctor Who, I think?). They have the same actor, but differ in personality, etc. due to some difference in their history. However, each Earth has integral events such as the particle-accelerator explosion that happen regardless of what Earth it is. This is explored more in "Crisis on Two Earths" (great DC animated movie, if you like that sort of thing) where Owlman discusses how each new Earth is created when a choice is made differently. Like, there's an Earth where Lex Luthor is good (one might call him an ALTERNATE version), one where life didn't happen at all, etc. [This concept seems to be universally geocentric throughout DC media, but whatever]. Contrastingly, Stephen King's "The Dark Tower" series mainly takes place in a world completely unrecognizable to our own, that runs 'parallel' to our own. Therefore, I'd call it a parallel world, based on its interaction and seeming disconnection to our world. [Maybe it will be revealed to be the future, but I'm not far into the series yet, ha.] A world that can't possibly be a variation of our own (a world where magic is real, the Dune universe with its melange, etc.) would typically be defined as parallel. These are just my interpretations, but I think they hold true throughout most media. The author of every work reserves the right to use the terms them interchangeably, or use one term solely. I have yet to encounter a work that uses both terms to mean a separate thing, but let me know if you find one (that would be a very interesting story!). – m-cubed8 years ago