Illegal alien, illegal, undocumented, and unauthorized are labels used to describe people unlawfully residing in the United States, whether by clandestine border crossing, visa deadline violation, or other means. But the terms are not interchangeable. Each has a unique origin and connotation.
Alien is a term used in legal language for a noncitizen resident, regardless of whether that person resides in the country legally or illegally. The term in the United States is inherited from British law, and has been a legal designation for foreign-born residents since the Revolutionary era. While the term continues to be used by the Department of Homeland Security and the United States Immigration and Customs Enforcement as a technical term in legal documents, it is not preferred in general discourse. As alien is a word that is also associated with extraterrestrial life, it is perceived as dehumanizing when applied to immigrants.
Illegal alien is a designation for unlawful immigrants that was previously widely used, but is now associated with anti-immigration policies and advocates.
Though the adjectival usage of illegal (illegal immigrant, illegal resident) may have an anti-immigration connotation, it is still largely accepted. However, as a noun, illegal is highly offensive. This noun implies deviance or emphasizes the criminality of an individual whose only crime would have been illegal entry into the country or an overstayed visa.
While there is a long history of immigration without documents or without papers, the adjectives illegal and undocumented were primarily associated with Chinese and Japanese immigrants in the late 19th and early 20th centuries. However, since the mid-20th century, these terms have become associated with people of Mexican and other Latin-American origin; they are even sometimes used to erroneously describe legal residents with proper documentation, based on their ethnicity.
It is more sensitive to use the adjectival forms undocumented or unauthorized when describing people: an undocumented immigrant , an unauthorized resident , etc. The use of these terms as adjectives reflects the legal status of the person without defining that person exclusively by immigration status.