WordNet records no synonym and no antonym for art nouveau. 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.
a French school of art and architecture popular in the 1890s; characterized by stylized natural forms and sinuous outlines of such objects as leaves and vines and flowers
Synonyms, antonyms and the broader and narrower relations for art nouveau come from Princeton University’s WordNet. Full sources and licences.