What is an Illegal ALIEN?

I here this phrase on a daily basis due to my close locality to the us-mexico border and it bothers me that the word alien is used instead of imigrant. back in the early 1900's i dont belive european imigrants where labled this way so why now? Dont we live in a country where political correctness is highly implmented by society! I hope someone out there can give me the true definition of an ALIEN becouse I always beleived they where little green men or beings of another world as depicted by discovery channel oo the history channel. If one is not sure what an iligal alien is maybe we should start calling them for what they really are. ILLIGAL IMIGRANTS!!!!!!!