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.

http://www.adversity.net/Terms_Definitions/TE...