WordNet records no synonym and no antonym for stamp act. 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.
an act passed by the British Parliament in 1756 that raised revenue from the American Colonies by a duty in the form of a stamp required on all newspapers and legal or commercial documents; opposition by the Colonies resulted in the repeal of the act in 1766
Synonyms, antonyms and the broader and narrower relations for stamp act come from Princeton University’s WordNet. Full sources and licences.