Naturalization
The conferring, by any means, of citizenship upon a person after birth.
Source: U.S. Citizenship and Immigration Services
Process by which a person acquires nationality after birth and becomes entitled to privileges of citizenship.
Source: U.S. Courts
A citizen who acquires nationality of a country after birth. That is, the person did not become a citizen by birth, but by a legal procedure.
Source: U.S. Department of State.