Citizenship of the United States
A person who is considered a citizen by more than one nation has "dual citizenship". It is possible for a United States citizen to have dual citizenship; this can be achieved in various ways, such as by birth in the United States to a parent who is a citizen of a foreign country (or in certain circumstances the foreign nationality may be transmitted even by a grandparent) by birth in another country to a parent(s) who is/are a United States citizen/s, or by having parents who are citizens of different countries. Anyone who becomes a naturalized U.S. citizen is required to renounce any prior "allegiance" to other countries during the naturalization ceremony; however, this renunciation of allegiance renunciation of citizenship to those countries.
Accidental American
U.S. law also states that a child born outside of the U.S to a U.S. citizen parent who previously spent sufficient time in the U.S. is a U.S. citizen at birth, regardless of whether the child also has the citizenship of the country of birth or another citizenship. U.S. citizens married to fellow U.S. citizens can transmit U.S. citizenship to their children if either parent has ever had a residence in the United States (without any minimum time limitation on how long they held that residence.) However, for U.S. citizens married to non-U.S. citizens, the required period of residence is longer; under the Nationality Act of 1940 and the Immigration and Nationality Act of 1952, the required period of residence was set to ten years, five of which had to be after the age of 14. The Immigration and Nationality Act Amendments of 1986 reduced this to five years, two of which had to be after the age of 14.