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 libelious is NOT synonymous with "negative" or "biased." Libel is not permitted here; bias and negativity are. However, biased pages should be equaled out with the opposing viewpoints; 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