The noun legitimacy means authenticity. If you found an old copy of the Gettysburg Address in your grandmother's attic, you'd have to question its legitimacy when you realized it was written on the back of a car advertisement.
Legitimacy comes from the Latin verb legitimare, which means lawful. Legitimacy, then, refers to something that is legal because it meets the specific requirements of the law. The legitimacy of various countries' presidential elections is often discussed, with monitors keeping track of the voting procedures to make sure they follow all the necessary rules.
