well-defined has 2 distinct synonyms spread across 2 of its 2 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 ill-defined.
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.
having a clean and distinct outline as if precisely cut along the edges
“a finely chiseled nose”
Synonyms, antonyms and the broader and narrower relations for well-defined come from Princeton University’s WordNet. Full sources and licences.