pacemaker has 6 distinct synonyms spread across 4 of its 4 senses. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. WordNet records no antonym for it — antonymy is a relation between word forms rather than concepts, so it is deliberately sparse.
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 leading instance in its field
“the new policy will be a pacesetter in community relations”
a specialized bit of heart tissue that controls the heartbeat
Synonyms, antonyms and the broader and narrower relations for pacemaker come from Princeton University’s WordNet. Full sources and licences.
an implanted electronic device that takes over the function of the natural cardiac pacemaker