text: what is the true definition of an illegal alien? i am writing a paper on it and i need the true meaning... an illegal alien is a foreign national who resides in another country unlawfully, by either entering that country at a place other than a designated port-of-entry or as result of the expiration of a non-immigrant visa. alternative terms include "illegal immigrant" and the euphemisms "undocumented immigrant", "undocumented worker", and "paperless immigrant".