WordNet records no synonym and no antonym for ballottement. 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 palpatory technique for feeling a floating object in the body (especially for determining the position of a fetus by feeling the rebound of the fetus after a quick digital tap on the wall of the uterus)
Synonyms, antonyms and the broader and narrower relations for ballottement come from Princeton University’s WordNet. Full sources and licences.