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.
Multiple citizenship
In former times, some countries (Latin American countries and Canada) advertised their policy of unconditional birthright citizenship to become more attractive for immigrants. Despite wide acceptance of dual citizenship, industrialized countries now try to protect themselves from birth tourism and uncontrollable immigration waves, so only Canada and the United States still grant unconditional birthright citizenship (even for children of illegal immigrants). In both countries, there have been calls to change the laws, but, so far, they have not been successful. Brazil has similar policies; the only people born in Brazil who do not automatically acquire Brazilian citizenship are those whose parents are residing in Brazil while serving their own countries (as diplomats, military attachés, cultural attachés and the like).