*sigh* Do we really have to explain?
So some people are thinking, "Oh no, not plagiarism definitions again!". And yeah, me too. The thing is, I see the same assumptions and faulty arguments again and again and I think these things should be corrected wherever and whenever possible.
Plagiarism - Taking someone else's work and/or words and claiming them as your own. Either explicitly or implicitly, either on purpose or by accident.
To reiterate, plagiarism doesn't care about intent. It's like stealing, even if you accidentally brush something into your purse and walk out of the store with it, you are stealing, though the unintentional offenders do often get leniency, in both cases. Actually, wait, plagiarism IS stealing. One of the largest problems I see is that some people cannot equate it with a loss of property because it is not something physical you can pick up and feel.
Doesn't matter. The theft of intellectual property is just as illegal and morally wrong as stealing physical property.
Next up.
EDIT: My appolgies, Copyright violation is the umbrella law, it encompasses all the terms we tend to talk about herein.
Plagiarism is a form of copyright violation, not ALL copyright violation is plagiarism.
Copyright violation is the larger law that most fanfiction tends to skirt around, near or into. We are using characters, places, events, plots that are not our own, and playing with them. The thing is that most fanfic acknowledges that these things are not ours and most readers (in fact I'd dare say all since most fanfiction is presented AS fanfiction with the universes labeled and everything) are aware that these things are not the original creations of the authors.
I am the worst offender of forgetting to add my own disclaimer, acknowledging that these characters aren't mine (and if I'd remember to make a damn template, this wouldn't happen nearly as often), however I label my show and post to show specific communities and archives. There is no way anyone could mistake the IDEAS AND CONCEPTS that belong specifically to that television show in my fanfiction as my own original works.
This above, is the basic principle of fanfiction. We all know that John and Rodney and the Stargate are not mine. Or that Ron and Harry are property of J.K. Rowling. I am not claiming they are anything but. I am possibly using them without permission, but I am not claiming them as my own.
Now if I were to say, incorporate large portions of Rowling's text, or Stargate dialogue into my stories and not attribute (cite) them then I am tacitly agreeing that yes, these are my words, I wrote them. That is wrong. It can be argued that a line or two, here and there (especially famous lines, things that become taglines and the like) don't need to be monitored as harshly and it's very easy not to even remember you have a specific line here or there, no one's going to come down on you like the high handed man from the plagiarism office for something like that.
It's all in the ratio's kiddies. How much of your story is your story? How much of it can be found word for word in another source. Personally I'd say there is a point you hit, that even with citation, you've crossed a line, but that's a personal preference, though I'd be happy to have someone more knowledgeable point out specific laws and/or rules.
In summation:
Fanfic writers are NOT (as a whole) claiming the ideas and concepts (basically the world they're playing in) in their stories are their own ideas and concepts. Plagiarists will claim other people's words, ideas and concepts as their own.
One final contradiction: Free domain.
I don't want to spend a lot of time on it but there are works (such as Shakespeare) in the Free Domain. This means that the copyright on their material has long since gone. You can still plagiarize from these works and it is still WRONG to take the words of authors whose works that fall into these categories and make them your own.