Is there a difference between a parallel universe and an alternate universe?

Parallel universe is a term describing a (theoretical) physical concept of having a similar yet somewhat different universe separate from the "given" one; this concept may be used as one of the elements in SFF setting.

Alternate universe is a term describing a literary (or other media) technique of pretending that an existing universe is somehow different.

Bear in mind that neither has an "official" definition in SFF and as such, it is quite possible that different works/authors can use either one with different connotations or even meanings.

Writers aren't very strict when it comes to these two particular terms. Most of the time, the two terms are interchangeable, unless they've been defined to be different within that continuity.

For example, in Star Trek, the word "parallel" is taken to mean "alongside", not that the two universes developed in parallel/mostly in parallel. They have timeline splits for universes that were identical up until the divergence point, an antimatter universe that appears to be otherwise completely identical to ours, and at least one "parallel universe" that's on a difference plane of existence.

When I think of parallel I see this:

When I think of alternate I see something like this:

What I'm heading at is that I think if a universe A is parallel to universe B then they were both created at the same time. And if universe C is alternate to universe D then either one of them was created first and the other one is a "spin off".

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Parallel universes will have same tendencies and/or directions as well as have comparable parts, some analogous aspects and/or recognized similarities.

I can only assume that alternate universes will change back and forth between some conditions and/or states, actions. I don't think that alternate universes would have same direction (I might be wrong about that so don't quote me).

This picture

should shed some light on this topic.

Think of an alternate universe as a line that as it progresses through time more and more branches "generate" from the "original" line. So each bifurcation represents a different outcome of any situation; for example, this morning you had the option to stay home or go to work, but you chose to go to work, an alternate universe would branch off from the decision of not going to work. A PERFECT example of this would be the "Butterfly Effect" movies, somehow the protagonists can travel between different lines of the same but ALTERNATE universe... furthermore, in that alternate universe there is not another version of the same person: this is another way how parallel and alternate universes are different; if you travel to a parallel universe you will find a slightly different version of yourself, a perfect example would be the series "Fringe". In Fringe when a protagonist travels to the parallel universe then he/she finds another version of the self (however in the series they sometimes refer to the parallel universe as alternate universe which is incorrect). A HUGE DIFFERENCE is that parallel universes (as the name suggests) NEVER touch each other, and if they do it would theoretically destroy one or both of the universes (as in Fringe); Alternate Universes generate from ONE SAME universe so at some point in time they were the same.

EDITED:

One more thing, Fringe got it wrong about Deja Vu's... Deja Vu's are nothing but the impression of recalling something not a window to an alternate universe. So what causes Deja Vu's? The best explanation I've read states that our brain uses a chemical compound when we are trying to recall something in order to help remember faster and better; when we have a Deja Vu it simply means that that chemical has been produced mistakenly when we were not trying to remember anything.

Sorry that I'm very late in this game, but here's my two cents about all this. Parallel universe and alternate universe can be distinguished, as seen above. But they could also be one and the same, at least in the realm of science fact. (Sorry if I'm posting this on a science fiction group.)

I'm having in mind the many-worlds interpretation and other multiverse interpretations in the field of quantum physics. In a nutshell, such hypotheses posit that all alternate or "what-if" histories, both that have already occurred and will occur in the future, take place in real life, just that they're in parallel universes or alternate universes. This way, "what-if" histories don't have to be restricted to the realm of a fictional alternate space in this universe - in other words, not restricted to the conditional, as in to the "could have been", "would be", etc. And multiverse theories could serve to reduce, if not eliminate, the suspension of disbelief in terms of alternate histories. It's possible that there's an infinite number of such universes in the cosmos.

Having said that, alternate histories or alternate universes could still be considered fictional, though often based on plausibility, in the sense that one can only approximate, based on real-life examples from our own universe, what such parallel/alternate worlds are like.

The terms are essentially interchangeable; some people may make distinctions, but there is no consensus.

The people behind the Historical Dictionary of Science Fiction have studied the usage of these terms (alternate reality, alternate world, parallel universe, etc.) more seriously than anybody else; they have considered most every combination of adjective and noun you can think of, and they have dredged a large body of literature for evidence. Here are their definitions (follow the links to see many historical examples of the use of each term):

alternate reality = alternate world

alternate universe = alternate world

alternate world one of many possible universes, which may have different physical laws or a different history than our own

alternative reality = alternate world

alternative universe = alternate world

alternative world = alternate world

parallel universe a universe conceived of as existing alongside our own, having many similarities to it but usually differing from it in some significant way (as having a different history or different physical laws)

parallel world a world that exists alongside or in addition to the known world; = parallel universe

If these three definitions are not exactly the same, the differences are obscure and insignificant. Jeff Prucher's Hugo-winning book Brave New Words: The Oxford Dictionary of Science Fiction is largely based on the material at the Science Fiction Citations site, precursor to the Historical Dictionary of Science Fiction. The definitions in Prucher's book differ slightly in wording but amount to the same thing:

alternative world n. one of many possible space-time continua, having a different history or physical laws than our own space-time continuum

parallel universe n. a universe or space-time continuum that exists alongside ours, and which may possess different physical laws or a different history. Also used fig. Compare PARALLEL WORLD.

parallel world n. a PARALLEL UNIVERSE.

Of the terms discussed here, only parallel universe and parallel world have entries in the online OED at this time:

parallel universe n. (a) (orig. and chiefly Science Fiction) a universe conceived of as existing alongside or in addition to our own, having many similarities to it but usually differing from it in some significant way; also fig.; (b) (in the many-worlds interpretation of quantum mechanics) a universe regarded as actually existing alongside our own, albeit with no possibility of detection (see many-worlds n. at MANY adj., pron., n., and adv. Compounds 2).

parallel world n. fictional or hypothesized world held to exist alongside or in addition to the known world; = parallel universe n.

To make it simple as possible, A Alternate Universe would simply be a universe where President Kennedy never gets killed or in 2016 the Warriors beat the Cavs. Another example would in this universe you decided not to go to work, in another Alternate Universe you decide instead to go to work.

A Parallel Universe would be instead Of humans becoming the dominant species, it would be the Neanderthals. Robert J. Sawyer book the Neanderthal Parallax is a good example of that. Another example would be Star Wars and Star Trek.

What about other Dimensions or better yet planes.

This is obviously more in fantasy but it also does occur in Science Fiction as well. Other Dimensions or planes, are essentially about beings living in a different plane of existence or realm. Different planes of existence is essentially different facets, frequency or Spectrum. This not the best example but let’s use Infrared light. Your eyes can not detect it but it does exist, certain sounds your ears cannot detect but the sounds indeed exist. So another plane of existence in Sci-fi or fantasy really is beings that vibrate at a different spectrum or frequency.

There would be many parallel universes, possibly a huge number, possibly very similar to ours or very different. But essential is that there are multiple ones. Travel between them may be possible. Time travel can be explained as going to a universe which is exactly like hours was 50 years ago or like hours might be in 50 years time.

If we talk about an alternate universe, there is/can be only one, and our universe cannot exist. It is essentially our universe, except it is more or less subtly different.