In the United States, a comprehensive discussion of what is and is not libel or slander is difficult, because the definition differs between different states....
...In the various states, whether by case law or legislation, there are generally several "privileges" that can get a defamation case dismissed without proceeding to trial. These include the litigation privilege, which makes statements made in the context of litigation non-actionable, and the allegedly defamatory statement being "fair comment and criticism", as it is important to society that everyone be able to comment on matters of public interest...
...On the federal level, there are no criminal defamation or insult laws in the United States.