noun
Refers to the quality of being legitimate, authentic, or orthodox, often in political, historical, or doctrinal contexts.
その政権の正統性が問われている。
The legitimacy of that regime is being questioned.
彼は自説の正統性
He asserted the orthodoxy of his own theory.
合法性 is specifically about legality under the law, whereas 正統性 is broader, covering historical or traditional legitimacy.
Compound of 正統 (seitō, 'legitimate, orthodox') and 性 (-sei, '-ness, -ity').