Legally, defamation was any false statement(s) that had the potential to harm an individual's reputation. "Slander" was the term for a spoken statement of this nature, while the term "libel" referred to a written one.
In 2378, while discussing the legal options that the crew of USS Voyager had regarding the stoppage of unauthorized distribution of an inflammatory version of The Doctor's holonovel Photons Be Free, Chakotay wondered if they could claim defamation. Tom Paris explained that they'd have to prove not only that they were represented by fictional characters in the holonovel, but that The Doctor's portrayals had harmed them. (VOY: "Author, Author")