WordNet records no synonym and no antonym for magistrate. 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 lay judge or civil authority who administers the law (especially one who conducts a court dealing with minor offenses)
Synonyms, antonyms and the broader and narrower relations for magistrate come from Princeton University’s WordNet. Full sources and licences.