I am confused regarding the usage of the tags and . Is there any clear guidance for their usage? Do we really need both the tags separately?

is used with 26 questions.

used with 37 questions.

4 questions have both the tags.

Is it not better to merge both the tags?

Note: Tags do make a difference.

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    All combinations of alternate/alternative/parallel reality/world/universe are synonymous. Some individuals may make distinctions, but there is no consensus. – user14111 Sep 30 '16 at 7:32
  • I just did the merge, and then found this question :-/ – Rand al'Thor Oct 6 '16 at 0:47
  • @Randal'Thor So what? The accepted answer to that question cites the Wikipedia definitions. The Wikipedia definition is not the definition, it's just a definition. As I said in my comment, some people make distinctions. – user14111 Oct 6 '16 at 3:58
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    @user14111 Thanks for your answer! I edited mine to say that there's no widely agreed-upon difference in meaning. – Rand al'Thor Oct 6 '16 at 13:23

UPDATE: merge and synonymisation complete.

You missed one: with 54 questions. Since there doesn't seem to be any widely agreed-upon difference in meaning between "parallel universe", "alternate reality", and "alternate world", and they're usually if not always used to mean the same thing:

I propose that all three of these tags should be merged & synonymised.

If this answer gets sufficient support, a mod can do this without bumping any questions to the front page. Please leave suggestions in the comments as to which of the three tags should be the 'master' and which should be its synonyms.

(There's also an tag. I believe "alternate history" may have different connotations and meaning to the other three - more about changing specific historical events than setting up an entire separate universe - but if I'm wrong, please do correct me in the comments and I'll edit the above to propose that all four tags should be merged and synonymised.)

Let's also take a quick look at the tag wikis. Some of them are better than others, and we need to decide which one(s) we want to keep elements from before doing the merge.


A reality different from that which is considered 'normal'. This normally occurs when timelines overlap and people/objects are transferred from one reality to another. E.G., consider the alternate reality in multiple Star Trek: The Next Generation episodes, including the alternate timeline of the final episodes 'All good things' - the events in all three timelines differ from what happens normally in the 'original' timeline.


A fictional location. Sometimes known as a parallel universe.

Alternate worlds are usually similar to, but are noticably different than the worlds they are viewed from. They often serve as cautionary tales and sometimes take the form of an alternate timeline (possibly from the result of time travel paradox's), or as a reality in another dimension. Any distortion of past history, depending on the the observers point of view, could be considered an alternate world.

A parallel universe or alternative reality is a hypothetical self-contained separate reality coexisting with one's own. A specific group of parallel universes is called a "multiverse", although this term can also be used to describe the possible parallel universes that constitute reality.

While the terms "parallel universe" and "alternative reality" are generally synonymous and can be used interchangeably in most cases, there is sometimes an additional connotation implied with the term "alternative reality" that implies that the reality is a variant of our own. The term "parallel universe" is more general, without any connotations implying a relationship, or lack of relationship, with our own universe. A universe where the very laws of nature are different – for example, one in which there are no relativistic limitations and the speed of light can be exceeded – would in general count as a parallel universe but not an alternative reality.

Fantasy has long borrowed the idea of "another world" from myth, legend and religion. Heaven, Hell, Olympus and Valhalla are all “alternative universes” different from the familiar material realm. Modern fantasy often presents the concept as a series of planes of existence where the laws of nature differ, allowing magical phenomena of some sort on some planes. One of the first science fiction examples is John Wyndham's Random Quest about a man who, on awaking after a laboratory accident, finds himself in a parallel universe where World War II never happened with consequences for his professional and personal life, giving him information he can use on return to his own universe.

Typically, parallel universes fall into two classifications. The first may be more accurately called a "diverging universe" whereby two versions of Earth share a common history up to a point of divergence. At this point, the outcome of some even happens differently on the two Earths and the histories continue to become more different as time elapses since that point, such as the episode "Parallels". The second type is where despite certain, often large, difference between the two Earths history and/or culture, they maintain strong similarities. In such cases, it is common that every person in one universe will have a counterpart in the other universe with the same name, ancestry, appearance, and frequently occupation but often a very different personality, such as the episode "Mirror, Mirror".

See also:

Further reading

Source: Parallel universe (fiction) on the English Wikipedia.

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  • I propose to make alternate-realities as the primary tag and better to keep alternate-history separate – Victor Salazar Sep 30 '16 at 13:14
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    My opinion would be, parallel-universe should stay as the master, as it is most common used term to my knowledge, and evidently, most common by the post counts as well. Wiki info is much better as well. alternate-history, while it does have significant overlap, is a sufficiently separate concept, and should not be merged. – Radhil Sep 30 '16 at 13:41
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    Another point: if alternate-history is going to be separate, then parallel-universe makes most sense as the primary tag simply because it's the least likely name to be confused with alternate-history! – Rand al'Thor Oct 1 '16 at 12:02
  • The second paragraph of parallel-universe is interesting, and I think I would have defined it the exact opposite - a "parallel" universe is one that has a connection to our own, containing many similarities, while an "alternate" universe/reality is something totally separate from our own (a "perpendicular" universe, if you'll excuse inventing terms). – Izkata Oct 3 '16 at 3:11
  • @Randal'Thor sound reasonable – Victor Salazar Oct 3 '16 at 4:57

The people of the Science Fiction Citations site 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 in which physical laws or historical events are different to our own

parallel world A world like our own, but which has [differences] in history or physical laws to our [own]

If these three definitions are not exactly the same, the differences are obscure and insignificant; nothing we need to take seriously, much less create separate tags for. 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. The definitions there 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.

On the other hand, alternate/alternative history is a horse of a different colour. Quoting the Science Fiction Citations site again:

alternate history a subgenre of science fiction wherein at least one aspect of history is different from that of our own world; the setting of such stories

alternative history = alternate history

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.

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