WordNet records no synonym and no antonym for greenmail. It is the only member of its synset, 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.
(corporation) the practice of purchasing enough shares in a firm to threaten a takeover and thereby forcing the owners to buy those shares back at a premium in order to stay in business
Synonyms, antonyms and the broader and narrower relations for greenmail come from Princeton University’s WordNet. Full sources and licences.