Defamation is saying or writing something about another person that hurts their reputation. Usually, when people use the word defamation, it means that the statement is not true. In this case, it is against the law. A statement that is defamation is called defamatory.
In order to be defamatory, a statement has to claim to be a fact instead of an opinion. For example, saying, "Mr. Jones is mean" cannot be defamatory, because it is someone's opinion. However, if someone says, "Mr. Jones steals candy from children", then the statement is defamatory if it is not true, because it would be a fact, not an opinion.
If a defamatory statement is written, such as in a book or in a newspaper, it is called libel. If it is spoken out loud, it is called slander. Somebody who is the victim of defamation can sue the person who wrote or said it.