noun: 1. the state or quality of being legitimate. Example: The legitimacy of the military dictatorship was not recognized by most other nations. Related Words: justice, authority, validity, right, lawfulness, permissibility, constitutionality. Synonyms: lawfulness, legality, rightfulness. Antonyms: illegality, illegitimacy, unlawfulness, wrongfulness. Origin: First recorded in 1685–95; legitim(ate) + -acy. Source 1, Source 2.