WordNet records no synonym and no antonym for concertina. 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.
coiled barbed wire used as an obstacle
free-reed instrument played like an accordion by pushing its ends together to force air through the reeds
Synonyms, antonyms and the broader and narrower relations for concertina come from Princeton University’s WordNet. Full sources and licences.