Well, this one's pretty much a look-it-up-in-the-dictionary, but since I've been referenced multiple times, I might as well do it for you.

slander: a false and defamatory oral statement about a person

libel: a statement or representation published without just cause and tending to expose another to public contempt

However, any legal definition is going to have to define ``published'', which I'm not qualified to do in such a context.