A shared universe or shared world is a series of fictions written by multiple authors in the same "universe" -- or using shared elements such as particular characterization, plot assumptions, etc. It can be argued that this concept is really as old as Arthurian legend or even mythology.
In professional fiction
Prominent examples of professionally-published shared universes include the Wild Cards series edited by George R. R. Martin and Melinda M. Snodgrass, the 1632 series created by Eric Flint, the Thieves' World series created by Robert Asprin, the Liavek anthologies edited by Emma Bull and Will Shetterly, and the Merovingen Nights books set in C. J. Cherryh's Alliance-Union universe.
Shared universes generally start off as a single story that inspires other fans to use the elements that set that original story apart from canon. Sometimes these shared universes are open for anyone to play in, and sometimes it's just a handful of fans, creating a universe together. Shared universes started very early in media fandom: the Kraith stories are a set of inter-connected works of Star Trek fan fiction. The earliest were written by Jacqueline Lichtenberg (also creator of the Sime-Gen Universe) beginning in 1969, in the T Negative fanzine, and continued through the first few years after the cancellation of the original TV series. As such, Kraith represents some of the earliest Star Trek fan fiction. By the time interest in Kraith had waned, over 50 writers had written stories in the universe.
In some fandoms, the shared universe can become as popular as the canonical universe, such as The Magnificent Seven fandom's widespread ATF universe, where the seven main characters are taken out of the Old West of canon and pulled forward in time to become modern-day ATF (U.S. Bureau of Alcohol, Tobacco, and Firearms) agents.
In other fandoms, shared universes occupy a niche, such as the Pegasus-B universe in Stargate Atlantis fandom, or the Less Than Legendary Journeys universe in Hercules: The Legendary Journeys fandom. In Pros fandom, an early elf story called The Hunting, by Jane of Australia, grew into a huge multivolume creation that nevertheless was only a small part of the larger Professionals fandom.
Some shared universes are multi-fandom, such as the stories set in Poisontaster's What We Keep universe or Helen's Take Clothes Off As Directed universe, which itself is a riff on Xanthe's Coming Home universe, which is a shared universe of its own.
Some Examples of Fan Authors' Invites
From the editor of the 1970's Star Trek zine, Echoes of the Empire: "In most ways, the universe is 'wide open.' So long as an action is effective and does not disrupt the Empire it is fine. It is strictly a 'whatever the traffic will bear' situation'."
From Lady Ra in 2010: "I know I have a tendency to create universes and then take FOREVER to get back to them. So here is an invitation to play in my universes. If you have a yen to create a timestamp for one of my sandboxes, just drop me a quick e-mail and let me know what your idea is. The rules are simple: keep my pairings, don't kill anyone off, and keep things happy! LOL. You know how to get a hold of me... I would love to read whatever you're thinking, and then we can link it to your live-journal, or wherever you want to post it." 
Some Examples of Fan Author's Invites, But with Many Stipulations
Some fanfic writers gave their official go-ahead for other fans to write in their universe or use their characters, but imposed many rules.
After she became too overcommitted to do so herself, Jacqueline Lichtenberg allowed others to write in Kraith. The rules and hoops for doing so, however, were staggering. The Kraith Round Robin was the platform. The round robin was intended to maintain the quality of writing and to monitor the continuity of the series. The round robin consisted of fans, mostly Kraith creators in their own right, who have touched upon some aspect of the story in question, or who have simply proven themselves to be capable writers and editors. The early round robins were small, but sometimes a story would go through a cycle of fourteen people, a process that could take many, many months, particularly if the story took on a radical or new concept. This meant the work would get tied down in letter arguments. There were many stories that were never finished due to the amount of time it took for approval. Sometimes the author was asked to make so many changes that she or he simply gave up on the story. The members of the round robin commented on everything from Kraith values and concepts, to plot line and story development, to paragraph organization and grammar. If a similar point of criticism occurred repeatedly in the letters, the story had to be rewritten and sent back to the robin a second time. Any changes in the final draft could only be made by Carol Lynn and Debbie Goldstein (the editors of Kraith Collected) and by Jacqueline Lichtenberg herself.
Another example of a sort of shared universe was with Marion Zimmer Bradley who said that she didn't so much create Darkover as she discovered it, and she encouraged fan fiction writers to write in, what she called, her "back yard." Marion, however, had ultimate control over what got written and where it was published, and was known for being very strict about other authors adhering to certain "realities" in her Darkover universe.
Ruth Kurz was the creator of the Harry & Johnny fandom using the characters Harry Callahan from Dirty Harry and Magnum Force and John Davis from Magnum Force. Ruth recruited many other authors to help write fiction in this very tightly-controlled universe. The lives of Harry and Johnny were thought out in every detail, and aside from a few planned instances, there was no overlap or contradiction in the fiction.
Jean Lorrah requested that fans not create further works in her Epilogue, but said fans are "heartily invited to write" in the universes of The Night of the Twin Moons and Full Moon Rising, though with some hoops to jump through:
There are plenty of other stories to be told that I don’t have enough time to tell. All I ask is if you want to write in the universe, please send me an outline before you start to write, so that if there is an inconsistency — which will probably be because I knew of something planned that hasn’t shown up in a story yet — then usually very minor tinkering can take care of that when the story is in outline form. 
Some Examples of Fan Author Bans
Not all fanfic writers were willing to share, even going so far as to threaten other fans with "copyright infringement."
Nu Ormenel (1978)In 1978, Fern Marder and Carol Walske wrote a personal statement complaining about copyright infringement on the characters/universe they had created:
Guinn Berger responded:There is a relatively new fanzine available, called Antithesis, which deals with the Klingon Empire. The first issue contains material by the zine's editor, P. Spath, which bears close resemblance to details of our Klingon series, Nu Ormenel. We have been assured by Ms. Spath that these similarities are purely coincidental and that she has drawn her material from other sources and her imagination. We are willing to accept her word for this. In view of this situation, however, we would like to make a few comments with reference to the 'Nu Ormenel' material. All works in the 'Nu Ormenel' series have been copyrighted to us, either directly or through the editors of the fanzines in which they appeared. Therefore, the use by any other authors of any 'Nu Ormenel' data about Klingons, our characters, original vocabulary, and proper names, is an infringement of our copyright and will be dealt with accordingly.  Any work regarding Klingons which is not identified as part of the 'Nu Ormenel' series, and/or which is not written by one or both of us, IS NOT part of 'Nu Ormenel.' We have not given our permission for anyone to write and publish stories in our series, nor do we plan to in the future. Any story which contains material from 'Nu Ormenel', be it labeled as such or not, has been published contrary to our wishes and to the copyright laws. 
Jeff Johnston commented on the idea of shared universes and copyright:Ordinarily I don't go in for 'foaming-at-the-mouth' LOCs, but I have just finished reading the personal statements section of the latest Scuttlebutt , and I'm about mad enough to chew neutronium. The creators of the Nu Ormenel series, it seems, are particularly peeved by the very suspicious (they say) similarities of Other Writers' Klingon stories to Nu Ormenel. These Other Writers have not now, and never will have their permission to write Nu Ormenel—and if This Situation goes on (they warn darkly) Appropriate Action will be taken. (!) Well, gee. I guess they mean they're going to hire a hit man! I hope they don't seriously believe the copyright laws will protect their Magnum Opus. Anyone who does more than hint that their story is about Klingons, or Andorians, or Romulans, or Thalosians, is violating the copyrights held by Gene Roddenberry, Paramount, Norway Productions, et al—disclaimers notwithstanding. This goes for the girls who are so fierce about protecting their version of Klingon civilization from rip-off, too. A theft is a theft, and we in fandom are all guilty. You cannot plagerize someone's work, expand upon it a bit, and then warn everyone else to keep their cotton-pickin' hands off because it's yours now. We all are lucky that the fan phenomenon is seen by the true holders of ST copyrights as a desirable thing—otherwise the "Appropriate Action" they'd take would have us up to our necks in legal fees. What I'm trying to say, I guess, is let's stop all this 'I-have-a-copyright-and-I'll-sue-your-ass' nonsense inside fandom. Sure, it's good manners to ask permission to use somebody's stories or even their ideas—but how many of us did that before publishing our ST stories based on the brainchild of Gene Roddenberry? HMMMMM? 
Fern Marder, co-creator of Nu Ormenel, commented on copyright:I too saw the comments in Scuttlebutt and it almost did appear that the authors of the Nu Ormenel series wanted to insure a monopoly on their own alternate universe by use of threat. I wonder if that was just a reaction on the part of the authors to keep the competition away. It seems to me that alternate universe series are becoming rather popular. Kraith is probably the first, but there have been many others since then. However, nothing guarantees that the characters that you decide to use for your alternate universe can't be used by others. There is a bit of paranoia involved on the part of some authors who may wish to have one whole segment of the Trek universe reserved just for them. 
In reference to [Guinn B's] letter (Interstat #9). There are two distinct issues raised here. Ms Berger has a valid point in her statements about the copyrights held by Gene Roddenberry, et, al. However, Mr. Roddenberry, on behalf of Paramount, etc., has given formal permission for the publication of fanzines dealing with Star Trek material. Therefore, it is legal for fans to write Star Trek-related stories. It is not a "rip-off." The provision of a copyright means only that one must have permission to use such material. Second, any writer, fan or otherwise, has the right to copyright as his or her own any original material created, even though it might have its distant roots in other material. A novel about the life of Neil Armstrong is copyrightable, is it not? The Nu Ormenel series is based on a large body of original material created by Carol Walske over a period of ten years. This includes historical background, social structure, biological data on the species discussed, and many, many characters. The stories in the Nu Ormenel series are based on this work. Our statement in Scuttlebutt was intended to protect the original material created for Nu Ormenel. There is a general feeling in fandom that anything in a fan-written story may be used by any other fan, as if it had appeared in Star Trek itself. Gene Roddenberry gave permission for Star Trek material to be used—we have not. Our long-range plans as writers make it Impossible to permit others to borrow from original Nu Ormenel material. We hope that our fellow fans—both readers & authors will understand our position & recognize our legal right to protect our work. 
Sahaj Universe (1978)Leslye Lilker addressed the unauthorized use by another fan of a word that Lilker had created:
I would like to answer a question that has been cropping up frequently in my mail since the publication of THE SENSUOUS VULCAN. In this 'zine appears a story entitled "The Way of a Warrior" by Karen Lewis. The author has used "Valjn'd'jt", the name of Sarek's home in the Sahaj universe, despite a request not to. I am not Karen Lewis (a pseudonym). I did read the story before publication, and, for my answer to "The Way of a Warrior", please read "Nivar to a Desert Rose", to appear in THE OTHER SIDE [#3]. Send s.a.s.e. to Amy Falkowitz [address redacted] for information. 
Epilogue (1989)In 1989, Jean Lorrah explained why she didn't want other fans to write in the Epilogue universe:
Some of you may be familiar with EPILOGUE, which is one of my TREK universes, which is a closed universe. That is, it is a complete novel in two volumes — eight chapters — and the eight chapters tell independent stories, with beginnings, middles and ends. But they are not really independent of one another, even though the first three appeared in TRISKELLION, way back in the early 1970’s. They don’t leave you satisfied, they leave you wanting the rest of the story, but it’s closed. There’s no more EPILOGUE. I will not write any more and no one else will write any more that in official EPILOGUE. You cannot prevent someone else from writing a story, but I will not recognize it—no matter how brilliant it is — as part of that universe because that one is complete... ... I was working off my own frustrations writing the first part of EPILOGUE. And then I came back and wrote part two after I had lived through and come to terms with that period of my life, and I had a book to complete. And then the artist goes on and finishes the story that was begun out of the emotion of a particular period. But it’s probably because it is so close to my own life that I don’t want anybody else fiddling around with it. It’s complete in itself. Now, that’s one kind of universe, where you as sole author, or perhaps with a collaborator write a particular thing, you finish it and that’s it. You don’t do any more. 
Sometimes writers using shared universes can result in problems or even wank, because there are different assumptions and expectations about how much an author can control a fanfic universe. Not every fan writer enjoys sharing their universes or having remixes made of their work. Some fans feel that any fan story is fair game to be used; others feel it should require specific permission until a shared universe is a certain (indeterminate) size, or has been declared open by the original author. An example for a controversy about different interpretations of a shared universe is the Clan Mitchell wank 2010.In 1989, one fan, Winston Howlett, explained:
Even when you open up your universe to other people, when you let them in, you still say, ‘Okay, but this has got to be twisted to fit right, because I’m still saying something, over all.’ And you’ve got to keep firm control over it. If you let it get out of hand, you wind up looking on it and saying, ‘What happened?!’ For a while, I had a fantasy where my universe would become like KRAITH, where I would have lots of other contributing writers. It didn’t happen, and now I look back on it and say, ‘Thank goodness it didn’t happen!’ because I know some of the problems Jacqueline Lichtenberg went through. 
Some fans feel a sequel shouldn't criticize the original, seeing it as other fans "fixing" or rewriting their work, while others think that it is hypocritical for fans to have more rights in their universes than they give the original copyright holders or creators of canon. Problems can arise when someone writing in a specific fan genre (slash, gen, het) finds their universe expanded to include a genre they may not approve of. Similar issues come up with the idea of fannish remixes.
- Alex Rider
- Forgotten Realms
- Good Omens
- Gravity Falls
- Harry & Johnny
- House, M.D.
- The Magnificent Seven
- Mary Renault
- Star Trek
- Star Trek: Deep Space Nine
- Star Wars: The Phantom Menace
- Stargate (SGA/SG-1 crossovers)
- Stargate Atlantis
- Stargate SG-1
- Visions of Pretty People Doing Naughty (Loving) Things*
- from a transcript of a writers' panel, published in Wulfstone, see complete transcript here, accessed March 6, 2013
- seeing how the 'Nu Ormenel' series was based on Klingons, someone else's copyrighted idea, seems to have escaped these two fan's attention
- from a personal statement in Scuttlebutt #7 (1978)
- from Interstat #9
- from Interstat #10
- from Interstat #10
- from Warped Space #31/32
- from a transcript of a writers' panel, published in Wulfstone, see complete transcript here, accessed March 6, 2013
- WebCite for Clan Mitchell on Journalfen.
- comment by Winston Howlett from transcript of a panel discussion in 1989, accessed March 6, 2013, also printed in Wulfstone