What is an Alien?

This definition comes from Section 1101(a)(3) of the Immigration and Nationality Act of 1952 (with amendments by Congress through 2001). An alien is "any person not a citizen or national of the United States." It is defined by United States statute. Aliens can be either legally or illegally in the U.S.

http://www.vdare.com/mann/faq.htm#report
http://caselaw.lp.findlaw.com/casecode/uscodes/8/chapters/12/subchapters/i/sections/section_1101.html