The statement harmed the reputation of the plaintiff, as opposed to being merely insulting or offensive. Generally speaking, a defamatory statement is a false statement of fact that exposes a person to hatred, ridicule or contempt, lowers him in the esteem of his peers, causes him to be shunned, or injures him in his business or trade. For more information, see the section on What is a Defamatory Statement.
The statement was published with some level of fault. Fault requires that the defendant did something he should not have done or failed to do something he should have. Depending on the circumstances, the plaintiff will either need to prove that the defendant acted negligently, if the plaintiff is a private figure, or with actual malice, if the plaintiff is a public figure or official. The level of fault that must be proven is discussed in the Actual Malice and Negligence section of the legal guide.
The statement was published without any applicable privilege. A number of privileges may be available depending on what the defendant published and the source(s) he relied on for the information. For more information, see the section on Defamation Privileges and Defenses in this guide.
In cases involving public officials, public figures or matters of public concern, a plaintiff must prove that the statement was false. In cases involving matters of purely private concern, in many states the burden of proving truth is on the defendant. This is not to say that every detail you publish must be perfectly accurate to avoid liability. If you get a few minor details wrong, this will not necessarily negate the truth of what you say so long as the statement at issue is substantially true. See the section on Substantial Truth for more information.
Statements of pure opinion, which cannot be proven true or false, cannot form the basis of a defamation claim (e.g., a statement that Bill is a jerk, is clearly a statement of opinion).