A statement against someone's reputation, which is intended to cause injury to that person. A defamatory statement can be libel (in a written form) or slander (in an oral statement).
Statement of Fact.
It must be a statement that is perceived as a statement of fact, not just an opinion. Opinions are generally protected under the First Amendment to the US Constitution as protected speech. A court must determine if a statement is truly an opinion, or a statement disguised as an opinion.
False statement about or concerning the person in question.
The statement of fact must be a false statement, because truth is a defense to defamation. And, it must concern the person in question, not a third party.
Published to a Third Person.
The statement of fact about the person in question, must be published (or relayed) to a third person. It's not enough that the person making the defamatory statement writes it in his/her journal, which no one will see, or makes the statement only to the person in question. When you make a defamatory statement on the Internet, by its very nature, that statement is communicated to third parties--many third parties.
Cause harm or injury to the person's reputation.
The final element for a successful defamation claim is there must be an injury. The person, against whom the statement is made, must suffer in some way, whether it's loss of reputation, loss of job, loss of friends, loss of livelihood, etc.