WordNet records no synonym and no antonym for central america. 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.
the isthmus joining North America and South America; extends from the southern border of Mexico to the northern border of Colombia
the nations of Central America collectively
Synonyms, antonyms and the broader and narrower relations for central america come from Princeton University’s WordNet. Full sources and licences.