The United States of America (USA), commonly known as the United States (U.S) or America, is a constitutional federal republic composed of 50 states, a federal district, five major self-governing territories, and various possessions.

A refugee, generally speaking, is a displaced person who has been forced to cross national boundaries and who cannot return home safely (for more detail see legal definition).

Such a person may be called an asylum seeker until granted by the contracting state or the UNHCR if they formally make a claim for asylum.

A security clearance is a status granted to individuals allowing them access to classified information (state or organizational secrets) or to restricted areas, after completion of a thorough background check.

The term "security clearance" is also sometimes used in private organizations that have a formal process to vet employees for access to sensitive information.

A clearance by itself is normally not sufficient to gain access; the organization must also determine that the cleared individual needs to know specific information.

No one is supposed to be granted automatic access to classified information solely because of rank, position, or a security clearance.

Immigration to the United States is the international movement of individuals who are not natives or do not possess citizenship in order to settle, reside, study or to take-up employment in the United States.

It has been a major source of population growth and cultural change throughout much of the history of the United States.

The economic, social, and political aspects of immigration have caused controversy regarding ethnicity, economic benefits, jobs for non-immigrants, settlement patterns, impact on upward social mobility, crime, and voting behavior.

The United States is the world's largest national economy in nominal terms and second largest according to purchasing power parity (PPP), representing 22% of nominal global GDP and 17% of gross world product (GWP).

The United States' GDP was estimated to be $17.914 trillion as of Q2 2015.

The U.S. dollar is the currency most used in international transactions and is the world's foremost reserve currency, backed by its science and technology, its military, the full faith of the US government to reimburse its debts, its central role in a range of international institutions since World War II and the petrodollar system.

Several countries use it as their official currency, and in many others it is the de facto currency.

The United States has a mixed economy and has maintained a stable overall GDP growth rate, a moderate unemployment rate, and high levels of research and capital investment.

Its seven largest trading partners are Canada, China, Mexico, Japan, Germany, South Korea, and the United Kingdom.