a. A false publication, as in writing, print, signs,
          or pictures, that damages a person's reputation.
       b. The act of presenting such material to the public.

Basically, libel is saying something false and damaging about someone in print. Don't do this, it is illegal and you can be sued. Libel is not to be confused with Slander.

Libel consists of three parts. First, the writer must publish an untruth about another person in a manner that is meant to be believed by others. Second, the object of the author's words must be damaged in some fashion. Third, to distinguish libel from slander, the publication must be in some form of permanent impression. That medium may be anything from a ponderous tome to a photograph either in a book or embedded in a website to an e-mail. Even an angry response posted to a chat room may be the subject of a lawsuit. However, since the wiki can be changed and altered almost instantly the permanency of a statement is questionable.

Furthermore, the most important component of libel is that the published material is untrue or misleading in a manner that defames the subject. If the words are facts, they cannot be libelous. Misinformation that is essentially harmless is also not considered to be defamatory and therefore libelous.

Problems arise when the published material is misleading or incorrect and when it also defames the subject. Then three issues come into play. First, was the subject a private or a public figure? Second, did the author show a reckless disregard for the truth? And third, did the author maliciously intend to do harm to the subject (only if the subject is a public figure—see the movie The People vs Larry Flynt) or was reasonable care taken (if the subject is a private party).

Note that "libelous" is NOT synonymous with "negative" or "biased." Libel is not permitted here; bias and negativity are. However, pages that present only one side can be balanced with other views if you have an opposing viewpoint; and negative information may not always be incorrect or even inappropriate.

Many insults are not considered libelous. For detail, see California Court of Appeal: "Dumb Ass" is not a defamatory term.

For more information, see the very comprehensive wikipedia entry

Libel on the Wiki

Please don't threaten the Wiki with libel lawsuits if another editor posts information you think is libelous. It's silly, it's obnoxious, and it's not going to help you get what you want. Most people don't like being threatened. The wiki, like all "interactive computer service" providers, receives wide protection from the Communications Decency Act. Your beef is with the person who wrote the potentially libelous statement.

That said, libel is not welcome here. The Wiki isn't set up to be some kind of arbiter of the truth; all of the editors here do their best to make sure obviously bogus claims don't stay. If you believe content is libelous, you should raise the issue. People will try to come up with a reasonable resolution, but you need to remember that people who aren't personally involved are often left trying to guess about a he-said-she-said situation. If the discussion doesn't turn out the way you want it to, you may need to take the issue up off the wiki. If you feel it's necessary, the Legal Services page can help you find local legal advice and/or representation.

It's also a bad idea to threaten fellow editors with libel lawsuits to shut them up. California has a broad anti-SLAPP (Strategic Lawsuit Against Public Participation) statute, viewable here. If you file a libel lawsuit against someone without enough information to show "a probability that [you] will prevail," and the alleged libel is speech in a public forum (like the Wiki!) about a matter of public interest, the judge can throw out your case and require you to pay the defendant's legal costs. That ain't cheap. If you're only threatening, just bear in mind that if you're reading this, the person you're threatening probably is, too. If they really are speaking falsehoods, you're probably better off taking formal steps to let them know you're serious. If you're just trying to intimidate them, you should stop, because they probably know it as well as you.

In short, if you actually believe you're being defamed and the community doesn't have enough information to agree the offending statement needs to be taken down, you need to address it through the proper channels. Don't be a jerk and throw around legal threats on here.