1. The fact or situation in which someone or something exists in accordance with the law or is valid: Joe entered the customs office and showed his passport to show his legitimacy to pick up the parcel that was waiting for him.

Because legitimacy was important when entering the country, Tim showed his passport to the officials on duty.
2. The condition of being in accordance with law or principle: Legitimacy is now usually referred to with respect to a sovereign's title or, in a narrower sense, the fact of being derived by regular descent.

Legitimacy also refers to the principle of lineal succession to the throne, as a political doctrine.
3. Conformity to rules or principles; that which is legal: Mrs. Thompson declared her legitimacy to enter the theater by showing her ticket that she had bought the day before.