WordNet records no synonym and no antonym for libel. It is the only member of each of its synsets, which is common for technical, taxonomic and single-sense vocabulary — there is no other English word that means the same thing. The broader and narrower terms below are the useful relations in that case.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
a false and malicious publication printed for the purpose of defaming a living person
the written statement of a plaintiff explaining the cause of action (the defamation) and any relief he seeks
print slanderous statements against
“The newspaper was accused of libeling him”
Synonyms, antonyms and the broader and narrower relations for libel come from Princeton University’s WordNet. Full sources and licences.