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.
British nationality law
A number of other countries do not allow multiple citizenship. If a person has British nationality and is also a national of a country that does not allow dual nationality, the authorities of that country may regard the person as having lost that nationality or may refuse to recognise the British nationality. British nationals who acquire the nationality of a country that does not allow dual nationality may be required by the other country to renounce British nationality to retain the other citizenship. None of this affects a person's national status under UK law.