defeasible has no recorded synonym. 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. Its one recorded opposite is indefeasible.
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.
capable of being annulled or voided or terminated
“a claim to an estate may be defeasible so long as the claimant is under 21 and unmarried”
Synonyms, antonyms and the broader and narrower relations for defeasible come from Princeton University’s WordNet. Full sources and licences.