legitimacy 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 illegitimacy.
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.
lawfulness by virtue of being authorized or in accordance with law
undisputed credibility
Synonyms, antonyms and the broader and narrower relations for legitimacy come from Princeton University’s WordNet. Full sources and licences.