Alien (law)

In law, an alien is any person (including an organization) who is not a citizen or a national of a specific country, although definitions and terminology differ to some degree depending upon the continent or region of Earth. More generally, however, the term "alien" is perceived as synonymous with foreign national.
Lexicology.
The term "alien" is derived from the Latin "alienus", meaning stranger, foreign, etym. "belonging (somewhere) else". Similar terms to "alien" in this context include "foreigner" and "lander".
Categories.
Different countries around the world use varying terms for aliens. The following are several types of aliens:
Common law jurisdictions.
An "alien" in English law denoted any person born outside of the monarch's dominions and who did not owe 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.
Australia.
In Australia, citizenship is defined in the Australian nationality law. Non-citizens in Australia are permanent residents, temporary residents, or illegal residents (technically called "unlawful non-citizens"). Most non-citizens (including those who lack citizenship documents) traveling to Australia must obtain a visa prior to travel. The only exceptions to the rule are holders of New Zealand passports and citizenship, who may apply for a visa on arrival according to the Trans-Tasman Travel Arrangement.
In 2020, in "Love v Commonwealth", the High Court of Australia ruled that Aboriginal Australians (as defined in "Mabo v Queensland (No 2)") cannot be considered aliens under the Constitution of Australia, regardless of whether they were born in Australia or hold Australian citizenship.
Canada.
In Canada, the term "alien" is not used in federal statutes. 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, and includes a stateless person."
United Kingdom.
In the United Kingdom, the British Nationality Act of 1981 defines an alien as a person who is not a British citizen, a citizen of Ireland, a Commonwealth citizen, or a British protected person. The Aliens Act of 1905, the British Nationality and Status of Aliens Act of 1914 and the Aliens Restriction (Amendment) Act of 1919 were all products of the turbulence in the early part of the 20th century.
United States.
Under the Immigration and Nationality Act (INA) of the United States, "[t]he term 'alien' means any person not a citizen or national of the United States." Certain green card holders who are not removable under the INA (i.e., not inadmissible under ) could at any time and anywhere in the world lawfully claim to being non-citizen nationals of the United States (Americans). This is especially true if such non-removable "legal immigrants" were originally admitted as small children and/or stateless refugees under , which basically means that they have no country (other than the United States) to reside in permanently, and also cannot lawfully obtain a passport of any country other than the United States. Such legal claim must be plausible and not frivolous because anyone who knowingly makes a false United States citizenship or nationality claim can be prosecuted and even removed from the country.
The correct alternative for "aliens" is "non-U.S. nationals ([or non-Americans])." The usage of the term "alien" dates back to 1798, when it was used in the Alien and Sedition Acts. Although the INA provides no overarching explicit definition of the term "illegal alien", it is mentioned in a number of provisions under title 8 of the US code. Several provisions even mention the term "unauthorized alien". According to PolitiFact, the term "illegal alien" occurs in federal law, but does so scarcely. PolitiFact opines that, "where the term does appear, it's undefined or part of an introductory title or limited to apply to certain individuals convicted of felonies." 
Since the 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.
"Alienage," i.e., citizenship status, has been prohibited since 1989 in New York City from being considered for employment, under that town's Human Rights legislation.
Other jurisdictions.
Arab states.
In the Gulf Cooperation Council (United Arab Emirates, Saudi Arabia, Kuwait, Oman, Bahrain, and Qatar), many non-natives have lived in the region since birth. However, these Arab states do not easily grant citizenship to non-natives. Most stateless Bedoon in Kuwait belong to indigenous northern tribes.
Latvia.
On Latvian passports, the mark "nepilsoņi" (alien) refers to non-citizens or former citizens of the Soviet Union (USSR) who do not 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 European Union and Russia, where latter is not possible for Latvian citizens.
References.
"This article is in most part based on the law of the United States, including statutory and latest published case law."