|Legal status of persons|
Different countries use varying terms for "aliens" including:
- an illegal immigrant, illegal alien or undocumented alien is person who is residing on a non-temporary basis in a country where s/he has no legal right to reside. It is a non-citizen who has entered a country through irregular migration, for example entered illegally, or an alien who entered a country legally but who has fallen "out of status." 
- a legal alien is a non-citizen who is legally permitted to remain in a country. This is a very broad category which includes tourists, guest workers, legal permanent residents and student visa resident aliens.
- a nonresident alien is a non-citizen who is visiting a country, for example as a tourist, on business, entertainers, sportspeople or in the country to receive medical treatment.
- a resident alien is a non-citizen who has permanent resident status in a country.
- an enemy alien is a non-citizen who is a national of an enemy country.
Common law jurisdictions
An "alien" in English law was someone who was born outside of the monarch's dominions and who did not have allegiance to the monarch. Aliens were not allowed to own land and were subject to different taxes to subjects. This idea was passed on in the Commonwealth to other common law jurisdictions.
In Australia, citizenship is defined in the Australian nationality law. Non-citizens living in Australia are either permanent residents; temporary residents; or illegal residents (technically called "unlawful non-citizens"). Most non-citizens (including those who lack citizenship documents) travelling to Australia must obtain a visa prior to travel. The only exceptions to this rule are members of the British royal family, and holders of New Zealand passports and citizenship who may apply for a visa on arrival according to the Trans-Tasman Travel Arrangement.
In Canada, the term "alien" is not used in federal laws and statues. Instead, the term "foreign national" serves as its equivalent and is found in legal documents. The Immigration and Refugee Protection Act defines "foreign national" as "a person who is not a Canadian citizen or a permanent resident". Permanent residents and Canadian citizens are not considered as foreign.
The British Nationality Act 1772 regulated who was to be called a British national.
U.S. law makes a clear distinction between aliens and immigrants by defining immigrants as a subset of aliens. Although U.S. law provides no overarching explicit definition of the term "illegal alien," the term is used in many statutes and elsewhere (e.g., court cases, executive orders). U.S. law also uses the term "unauthorized alien". U.S. immigration laws do not refer to illegal immigrants, but in common parlance the term "illegal immigrant" is often used to refer to any illegal alien.
Because U.S. law says that a corporation is a person, the term alien is not limited to natural humans because what are colloquially called foreign corporations are technically called alien corporations. Because corporations are creations of local state law, a foreign corporation is an out-of-state corporation.
There are a multitude of unique and highly complex U.S. domestic tax laws and regulations affecting the U.S. tax residency of foreign nationals, both nonresident aliens and resident aliens, in addition to income tax and social security tax treaties and Totalization Agreements.
In Arab States of the Persian Gulf (UAE, Saudi Arabia, Kuwait, Oman, Bahrain and Qatar), many foreigners have lived in the country since birth or since independence. However these countries do not accord citizenship to many of them. As such, referring to these people as foreigners is seen by some as rude.
On Latvian passports, alien refers to non-citizens (nepilsoņi): former citizens of USSR who don't have voting rights for the parliament of Latvia but have rights and privileges under Latvian law and international bilateral treaties, such as the right to travel without visas to both the EU and Russia, which is not possible for Latvian citizens.
- Alien and Sedition Acts
- Alien land laws
- California Alien Land Law of 1913
- Illegal immigration
- Multiple citizenship
- Non-citizens (Latvia)
- Persona non grata
- Public international law
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- "Immigration policy - EU fact sheets - European Parliament". Retrieved July 3, 2016.
- "Immigration Terms and Definitions Involving Aliens". Retrieved July 3, 2016.
- William Blackstone, Commentaries on the Laws of England (1753), Book 1, Chapter 10
- Key Issue 5. Citizenship Fact Sheet 5.2 Citizenship in Australia Retrieved 2012-03-05.
- "Australia's Visitor and Temporary Entry Provisions" (PDF). Joint Standing Committee on Migration, Parliament of Australia. 27 September 1999. Archived (PDF) from the original on June 29, 2011. Retrieved 20 July 2011.
- Immigration and Refugee Protection Act (S.C. 2001, c. 27)
- section 51, British Nationality Act 1981
- "8 USC 1101". .law.cornell.edu. Retrieved 2017-04-06.
- "Alien and Sedition Acts". Ourdocuments.gov. Retrieved November 23, 2011.
- "2 USC 658". Cornell University Law School. February 22, 2011. Retrieved 2017-04-06.
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- "42 USC 6705". Cornell University Law School. Retrieved 2017-04-06.
- "49 USC 40125". Cornell University Law School. November 1, 1999. Retrieved 2017-04-06.
- "8 USC 1188". Cornell University Law School. June 1, 1986. Retrieved 2017-04-06.
- "8 USC 1255". Cornell University Law School. Retrieved 2017-04-06.
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- Howell, Deborah (March 2, 2008). "Immigration Coverage in the Crossfire". The Washington Post.
- "Foreign Nationals: Non-Resident Aliens and Resident Aliens". Protax Consulting Services.
- Habboush, Mahmoud. "Call to naturalise some expats stirs anxiety in the UAE".
- "Say no to expats calling for Saudi citizenship". November 24, 2013.
- "GCC Citizenship Debate: A Place To Call Home - Gulf Business". January 5, 2014.
- Resident Alien Information
- D. C. Earnest Noncitizen Voting Rights: A Survey of an Emerging Democratic Norm, 2003
- University of Minnesota Human Rights Center: Study Guide: The Rights of Non-Citizens, 2003
- UK Aliens' registration cards on The National Archives' website.
- Council of Europe Commission for Democracy through Law Report on noncitizens and minority rights, 2007
- The rights of non-citizens, UN OHCHR, 2006