Definitions

Alien, Immigrant, Illegal Alien, Undocumented Immigrant

These related terms are often used in deliberately confusing and conflicting ways.  Here is a set of definitions that will help you sort out the difference.

IMMIGRANT:  In popular usage, an "immigrant" is generally understood to be a person who migrates to another country, usually for permanent residence.  Under this definition, therefore, an "immigrant" is an alien admitted to the U.S. as a lawful permanent resident.   The emphasis in this definition is upon the presumptions that (1) the immigrant followed U.S. laws and procedures in establishing residence in our country; (2) he or she wishes to reside here permanently; and (3) he or she swears allegiance to our country or at least solemnly affirms that he/she will observe and respect our laws and our Constitution.

ALIEN:  By contrast, an "alien" is generally understood to be a foreigner -- a person who comes from a foreign country -- who does not owe allegiance to our country.

ILLEGAL ALIEN:  An "illegal alien" is a foreigner who (1) does not owe allegiance to our country; and (2) who has violated our laws and customs in establishing residence in our country.  He or she is therefore a criminal under applicable U.S. laws.

The term "illegal alien" is used by U.S. citizens who believe that non-citizens entering our country must comply with our immigration laws. 

The term "illegal alien" is predicated upon U.S. immigration law which requires foreigners entering the U.S. to comply with our country's rules and laws regarding entry into, and residence within, our country.

UNDOCUMENTED IMMIGRANT:  The term "undocumented immigrant" is an oxymoron (the parts conflict). An immigrant is synonymous with "permanent legal resident." The old term for the document authorizing a permanent legal resident is "green card." The term "undocumented" is derived from the accurate term "undocumented aliens" who are often called "border crossers."

Proper terms are "illegal alien" or "undocumented alien" but not "undocumented immigrant." Although not commonly used, the term "documented alien" accurately refers to foreign nationals who have an unexpired non-immigrant visa such as H-1B.

Most U.S. citizens do not use the term "undocumented immigrant" and prefer, instead, the more descriptive and accurate term "illegal alien".

The term "undocumented immigrant" is used by those who believe in "open borders", i.e., non-regulation of foreigners entering into and assuming residence in the U.S., including even those foreigners who owe allegiance to a foreign government and/or who may intend harm to the U.S.

DISCUSSION:

Commonly, in U.S. politics and U.S. news stories, the term "alien" is most often used with the modifier "illegal".

By contrast, the term "immigrant" is often deliberately used without a modifier, leaving the impression -- correct or not -- that the immigrant is legally or permissibly in the U.S. under our laws.  Of course, this is frequently not the case!

Liberal politicians and news organizations prefer to refer to illegal aliens as "undocumented immigrants" because this term deliberately deflects discussion away from the fact that such individuals are in our country illegally.  The term "undocumented immigrant" is designed to deliberately gloss over the fact that such individuals have broken our laws.

Conversely, conservative and libertarian politicians and news organizations tend to refer to illegal aliens as "illegal aliens" which properly keeps the focus of the discussion on the fact that such individuals have broken our laws and are therefore criminals to one degree or another, and that these individuals do not owe allegiance to our country.

In fact, folks who like to use the term undocumented immigrant also tend to believe we should be spending U.S. taxpayer funds to provide illegal aliens (criminals) with health care, social security and in some cases in state tuition at our colleges and universities.  They also tend to believe that illegal aliens should be able to obtain U.S. drivers' licenses, open U.S. bank accounts, and be free from arrest and deportation by our police officers.