WordNet records no synonym and no antonym for cordon. 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 series of sentinels or of military posts enclosing or guarding some place or thing
cord or ribbon worn as an insignia of honor or rank
adornment consisting of an ornamental ribbon or cord
Synonyms, antonyms and the broader and narrower relations for cordon come from Princeton University’s WordNet. Full sources and licences.