improved 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 unimproved.
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.
made more desirable or valuable or profitable; especially made ready for use or marketing
“new houses are springing up on an improved tract of land near the river”
2 of the 3 recorded senses carry no synonym, antonym or related term, so they are not shown above. Every sense, with its definition and examples, is on the full entry for improved.
Synonyms, antonyms and the broader and narrower relations for improved come from Princeton University’s WordNet. Full sources and licences.