Wikipedia--Multiple Citizenship

Multiple citizenship is a status in which a person is concurrently regarded as a citizen under the laws of more than one state. Multiple citizenships exist because different countries use different, and not necessarily mutually exclusive, citizenship requirements. Colloquial speech refers to people "holding" multiple citizenship but technically each nation is making a claim that this person be considered its national. For this reason it is possible that a person be a citizen of one, none or many countries. more

Dual Nationality/Dual Citizenship

The concept of dual nationality means that a person is a citizen of two countries at the same time. Each country has its own citizenship laws based on its own policy.Persons may have dual nationality by automatic operation of different laws rather than by choice. For example, a child born in a foreign country to U.S. citizen parents may be both a U.S. citizen and a citizen of the country of birth. more

From United States Department of State

Am I a Citizen of the United States?

Many people are citizens of America by birth or through some other part of the immigration law and don’t even know it. If you are a citizen by law, you do not need to take the citizenship and naturalization test through the INS or U.S. Citizenship and Immigration Services. Even if you have no documentation or consider yourself an “illegal alien”, if you are citizen by law, your lack of documentation does not change the fact that you are a citizen – you just need proof of it. More

by Vincent Martin Immigration Attorney