The really LONG definition(s):
The definition of libel depends upon whom is asked. It is clear that the definition of libel in U. S. law revolves around defamation of character in the written form.
One definition of libel may not be enough to satisfy most, so we've put together the top 10 definitions of libel from various legal resources.
Definition of libel 1: Libel is the written act of defamation, vs. slander, the oral act of defamation.
Definition of libel 2: use of print or pictures to harm someone's reputation. Until 1964, a person could prove that they had been libeled simply by showing that the statements in question were incorrect. In 1964, the Supreme Court decided that public officials had to prove that the statements in question were made with "actual malice"-for the purpose of harming the person's reputation. As a result of the Supreme Court case, Time, Inc. v. Firestone (1976); private individuals only have to prove negligence, rather than "actual malice," on the part of the press.
Definition of libel 3: Defamation of an individual or individuals in a published work, with malice aforethought. In litigation, the falsity of the libelous statements or representations, as well the intention of malice, has to be proved for there to be libel. In addition, financial damages to the parties so libeled must be incurred as a result of the material in question for there to be an assessment of the amount of damages to be awarded to a claimant. This is contrasted to slander, which is defamation through the spoken word.
Definition of libel 4: Libel per se describes statements, which are widely understood to be harmful to a person's reputation. For example, referring to an individual as an alcoholic or criminal, or any description, which would lower the reputation of that individual in the eyes of others. These words are harmful and libelous.
Definition of libel 5: A written, printed, or pictorial statement that unjustly defames someone publicly. Prosecution of libel as a punishable offense puts some measure of restriction on freedom of the press under the First Amendment.
Definition of libel 6: To defame, or expose to public hatred, contempt, or ridicule, by a writing, picture, sign, to lampoon. A tort consisting of false and malicious publication printed for the purpose of defaming a living person.
Definition of libel 7: To publish in print writing or pictures, broadcast through radio, television or film something that is false about someone else which would cause harm to that person or his/her reputation by bringing the target into ridicule, hatred, scorn, or contempt of others. Libel is defamation, which is written, or broadcast and is distinguishable from slander, which is oral defamation.
Definition of libel 8: A malicious publication expressed either in print or in writing, or by pictures, effigies, or other signs, tending to expose another to public hatred, contempt, or ridicule. Such publication is indictable at common law.
Definition of libel 9: Published material meeting three conditions: The material is defamatory either on its face or indirectly; The defamatory statement is about someone who is identifiable to one or more persons; and, The material must be distributed to someone other than the offended party; i.e. published; distinguished from slander.
Definition of libel 10: A publication without justification or lawful excuse, which is calculated to injure the reputation of another by exposing him to hatred, contempt or ridicule.
As one can see, the legal definition of libel revolves around the maliciousness of the act and the harm it has causes. The act of libel, according to the law, has to be in written or visual form such as an article or photograph and has to somehow damage the reputation of a person or business in some way where the courts are the natural place to resolve the claims.
For example, if you were to write and publish an article accusing someone without a criminal record or any other proof of being a "disgusting pedophile" this statement would be very libelous. The person's reputation would be damaged and you would be open for a libel suit. If, however, you reported that you observed someone with a past criminal record of inappropriate behavior towards children, in a public place engaging children with candy and games and wondered if this was legal, then this would be a much safer statement.
In a libel case, the hardest part is trying to interpret the intent of the defendant. If someone's intent was clearly malicious, then a libel case has a good chance of succeeding. Libel is not libel when it is about and un-definable group of people or organization. Saying all "CEO's are crooks" is not libel, but specifically naming a person who is a CEO, most likely is.
The best defense in any libel case is "truth" as this element is thought to be something that mutual excludes libel. The concept of "truth" is different from "fact"
Wow, straight to the point. Really, I'm not concerned about him specifically (for the reasons you've mentioned). And for the record I have not ever been threatened with a suit for any reason, much less a review. The situation (and others this year with authors having not-glowing reviews removed from Amazon and harassing reviewers, etc.) has me thinking about the legalese of it. So I came here to ask people who are more knowledgeable than I. 
