WordNet records no synonym and no antonym for imperialism. 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 policy of extending your rule over foreign countries
a political orientation that advocates imperial interests
any instance of aggressive extension of authority
Synonyms, antonyms and the broader and narrower relations for imperialism come from Princeton University’s WordNet. Full sources and licences.