established has 5 distinct synonyms spread across 4 of its 5 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 unestablished.
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.
brought about or set up or accepted; especially long established
“the established social order”
settled securely and unconditionally
“that smoking causes health problems is an accomplished fact”
introduced from another region and persisting without cultivation
1 of the 5 recorded senses carries no synonym, antonym or related term, so it is not shown above. Every sense, with its definition and examples, is on the full entry for established.
Synonyms, antonyms and the broader and narrower relations for established come from Princeton University’s WordNet. Full sources and licences.