Any statement, whether written or oral, that injures a third party's reputation. See, e.g. Buckley v. Fitzsimmons, 509 U.S. 259 (1993).  The tort of defamation includes both libel and slander.

To establish a prima facie case of defamation, four elements are generally required:  a false statement purporting to be fact concerning another person or entity; publication or communication of that statement to a third person; fault on the part of the person making the statement amounting to intent or at least negligence; and some harm caused to the person or entity who is the subject of the statement.