The term alien can actually refer to either individuals or companies (such as a corporation). Alien (or foreign) corporations can refer to corporations from a different country or simply a corporation from a different state. For example, a Japanese corporation that operates in the United States is referred to as an alien (or foreign) corporation in the United States. Also, a New York corporation that operates in California will generally be referred to as an alien (or foreign) corporation in California.

The term alien is most often used in the context of immigration and naturalization law when dealing with individuals (i.e. becoming a citizen of country). Individuals become aliens in different countries for many different reasons such as due to war, political or religious freedom, poverty, family reunion, and economic opportunity. Aliens in this context can only be individuals (i.e. they cannot be companies) and may live legally or illegally in their host country. In other words, aliens can be either legal aliens or illegal aliens.

A legal alien is legally permitted to remain in a country. Individuals gain legal alien status for many different reasons such as guest workers, tourists, green card permanent residents, and student visa aliens. Legal aliens can further be subdivided into resident and nonresident aliens. A resident alien is an individual who has acquired temporary or permanent residence in the host country. A nonresident alien is an individual who is simply visiting the host country (e.g. a tourist).

An illegal alien is a person present in a country without that country’s authorization (i.e. against that country’s law). Illegal aliens are also referred to as undocumented persons.

Countries can grant different rights to aliens based on their own laws. For example, in the European Union (EU), aliens who are citizens of any EU country have the same rights to live, work, and travel in any EU country as in their home EU country. This is part of the EU’s effort to promote the free movement of individuals between member EU countries.

In the United States, the first law to deal with aliens was passed in 1798 through the Alien and Sedition Acts. Today, aliens must take certain legal steps in order to gain legal alien status. Then, legal aliens can work to eventually become U.S. citizens – a process called naturalization.