preemption has 1 distinct synonym 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.
the judicial principle asserting the supremacy of federal over state legislation on the same subject
the right of a government to seize or appropriate something (as property)
a prior appropriation of something
“the preemption of bandwidth by commercial interests”
Synonyms, antonyms and the broader and narrower relations for preemption come from Princeton University’s WordNet. Full sources and licences.