There are four criteria used today in the United States:
- The statement was false, but was claimed as true.
- The statement must have been made to a third, previously uninvolved party.
- The statement must have been made by the accused party.
- The statement caused harm.
The first (and very important) criterion was discussed in New York Times v. Sullivan, where it was ruled that
A State cannot, under the First and Fourteenth Amendments, award damages to a public official for defamatory falsehood relating to his official conduct unless he proves "actual malice" -- that the statement was made with knowledge of its falsity or with reckless disregard of whether it was true or false. Pp. 265-292.
(c) Factual error, content defamatory of official reputation, or both, are insufficient to warrant an award of damages for false statements unless "actual malice" -- knowledge that statements are false or in reckless disregard of the truth -- is alleged and proved. Pp. 279-283.
Quoting Wikipedia and Justice Black,
The Court held that a public official suing for defamation must prove that the statement in question was made with actual malice. In this context, the phrase refers to knowledge or reckless lack of investigation, rather than its ordinary meaning of malicious intent. In his concurring opinion, Justice Black explained that "'[m]alice,' even as defined by the Court, is an elusive, abstract concept, hard to prove and hard to disprove. The requirement that malice be proved provides at best an evanescent protection for the right critically to discuss public affairs and certainly does not measure up to the sturdy safeguard embodied in the First Amendment."
New York Times v. Sullivan is regarded as one of the most - of not the most - important defamation cases of the century. It was argued in 1964. If the case you discuss - which I haven't been able to find - occurred after to the ruling, then it could have been dismissed, because A did not intend it as malicious in the sense of defamation (and did not claim it was true), though it was almost certainly meant as an insult. Had this case occurred prior to New York Times v. Sullivan, things might have been different.
Things are different for private officials.
Those who are not classified as public figures are considered private figures. To support a claim for defamation, in most states a private figure need only show negligence by the publisher, a much lower standard than "actual malice." Some states, however, impose a higher standard on private figures, especially if the statement concerns a matter of public importance. You should review your state's specific law in the State Law: Defamation section of this guide for more information.
Note: There are differences between defamation, libel, and slander; a quick overview is given here: "Generally speaking, defamation is the issuance of a false statement about another person, which causes that person to suffer harm. Slander involves the making of defamatory statements by a transitory (non-fixed) representation, usually an oral (spoken) representation. Libel involves the making of defamatory statements in a printed or fixed medium, such as a magazine or newspaper."