There are two kinds of defamation: slander (verbal defamation) and libel (written defamation). So what is “defamation?”
There are 2 types of defamation: defamation of a private person and defamation of a public person. (I’m *really* leaving out a lot of subtleties here, subtleties that are incredibly important.)
For public persons as plaintiffs, the standard is “actual malice” — that the defendant knew what he communicated was false or exhibited ‘reckless disregard for the truth’ — i.e. didn’t even try to check it out. For private people, the standard is mere negligence. Obviously, then, the public/private question becomes a threshold matter. If you’re found to be a public person (i.e. in cases where there’s actually some question as to whether someone is sufficiently well-known), you can pretty much kiss your case goodbye because actual malice is a really high hurdle.
If you are a private person, or a public person where your case can past the “actual malice” test, then defamation is
(1) publication (i.e. communication to a third party — could be one person or a whole world full of people);
(2) that the communication is about the plaintiff (i.e. s/he is identifiable. Even if the person’s name isn’t used, if s/he is sufficiently described that people can figure it out, that counts);
(3) Plaintiff suffers actual financial (or other) damages which you must prove;
(4) the communication must be FALSE (this is the element upon which many libel suits fail); and
(5) the defendant must be at fault (intentional or negligent conduct).