WordNet records no synonym and no antonym for radicalism. 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.
the political orientation of those who favor revolutionary change in government and society
Synonyms, antonyms and the broader and narrower relations for radicalism come from Princeton University’s WordNet. Full sources and licences.