illegitimacy has 2 distinct synonyms spread across 1 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 legitimacy.
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.
the status of being born to parents who were not married
unlawfulness by virtue of not being authorized by or in accordance with law
Synonyms, antonyms and the broader and narrower relations for illegitimacy come from Princeton University’s WordNet. Full sources and licences.