establish has 18 distinct synonyms spread across 8 of its 8 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. Its one recorded opposite is abolish.
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.
establish the validity of something, as by an example, explanation or experiment
“The experiment demonstrated the instability of the compound”
Synonyms, antonyms and the broader and narrower relations for establish come from Princeton University’s WordNet. Full sources and licences.