This article is from the Alien Visitors FAQ, by CRAM: The Cyberspatial Reality Advancement Movement firstname.lastname@example.org with numerous contributions by others.
According to my dictionary :
ALIEN - belonging to another : a foreign-born resident of a country
in which he is not naturalized
Therefore the term alien means, more or less, somebody or something that is
in the wrong place. It should be noted that whilst extraterrestrial beings
might be a sub-group of aliens, not all aliens are (necessarily) extra-
terrestrial. Some people use the term 'space aliens' to refer explicitly
to extraterrestrial beings.