WordNet records no synonym and no antonym for theremin. 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.
an electronic musical instrument; melodies can be played by moving the right hand between two rods that serve as antennas to control pitch; the left hand controls phrasing
Synonyms, antonyms and the broader and narrower relations for theremin come from Princeton University’s WordNet. Full sources and licences.