Plain and simple, plagiarism is theft. It is the taking of someone else's work and presenting it as if it were your own.
This post is prompted by the discovery that two of my fics were plagiarized recently. [See
this post for details.] The plagiarist has claimed in response that this wasn't really plagiarism, that there are only so many ways to describe sexual activity between the same two characters and that therefore any resemblances between his fic and mine are simply coincidental.
Rubbish.
I'm a teacher. I regularly assign my students papers to write, and I warn them about plagiarism and how to avoid it. There are two basic principles to follow, and they're pretty much the same whether it's writing nonfiction research papers for a class, or a fanfic to post online.
1) Use your own words.
Like the way that someone depicted something? Want to borrow an idea they had and put it into your own piece? Not a problem, but make it your own. Conjure up that same image and think of a new way to describe it. The whole point of writing fanfic, really, is to tell a new story about beloved characters. If you simply re-use someone else's words, why bother?
Yes, certainly, there are a limited number of words in the language. But it's a pretty large number, and they can be put together in a very large variety of ways. Be creative!
Taking someone else's sentences, changing a few words here and there, and presenting them as your own is plagiarism. Start from scratch; don't have another fic open before you as "inspiration" or whatever. It's one thing to have a phrase stick in your head, and pop out again in your own writing. Then it'll usually be in a different context with a different purpose, and that's not such of a big deal. But lengthy passages, whole paragraphs that closely resemble someone else's -- those can't come from anything but outright theft.
Don't steal. Use your own words.
2) Acknowledge your source.
We're all writing fanfic here, and so some people seem to take the attitude, "It's all borrowed from the original author, and that's okay, so I can 'borrow' from another fanfic author too."
Not so. We disclaim regularly, stating that recognizable characters and other elements are the creations of the original author, not our own. The same applies to borrowing from another fanfic author; if I were to pick up on
resonant8's idea of wizarding playing cards that talk to each other and the player (and/or the game of Stealth as she describes it in
Transfigurations), for instance, it would be proper form to acknowledge that as her original idea, not mine.
In writing history, the rule is that information that is general knowledge need not be attributed. I wouldn't have to say where I found the information that Alfred the Great was the king of Wessex; that's common knowledge. Similarly in fanfic a common idea -- e.g. pairing Harry with Draco, or the notion that Severus' father was abusive -- doesn't require attribution. Lots of people can come up with those ideas independently.
Using specific phrases, though, is another matter. In writing a research paper you use quotation marks and footnotes to indicate that you've borrowed a whole phrase/sentence/paragraph. In fanfic, you should do the same thing, or at least mention the fact in an author's note.
Don't steal. Acknowledge your source.