The term "Alien" is derived from the
Latin alienus, meaning stranger, foreign,
etym. "belonging (somewhere) else".
Different countries use varying terms for "aliens" including:
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 illegal migration,
 for example entered illegally, or an alien who entered a country legally but who has fallen "out of status",
 and subsequently refused to leave the country as legally required, or a descendant of one.
- 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.
enemy alien is a non-citizen who is a national of an enemy country.