According to U.S. Citizenship and Immigration Services, a naturalized citizen is a foreign individual who has fulfilled the citizenship requirements established by the Immigration and Nationality Act. This act was passed by Congress in 1952 and contains four scenarios under which an individual can become naturalized.
An individual can become a naturalized citizen after being a permanent resident of the United States for at least five years. He can also become naturalized after being a permanent resident for at least three years and the spouse of a United States citizen. Those who served in the military are also eligible, along with individuals who were born outside of the United States but have one or more parents who are American citizens.