An alien is any individual who is not a citizen or a national of the U.S. Within the category of alien are several distinctions:
Legal alien: one who is permitted to remain in the U.S. based on certain terms
Resident alien: one who is permitted to reside in the U.S. either temporarily or permanently
Nonresident alien: one who is permitted to visit the U.S.
Illegal alien: one who resides in the U.S. unlawfully, either as a result of illicit entry or an excessively long stay that outlasts the duration granted by a non-immigrant visa