A foreign-born resident of a country. An alien can be in the country legally (often to work or seek an education) and eventually seek citizenship, if he or she lives there long enough, or he can be in the country illegally (usually to work or visit friends or family), which means he may be deported back to his home nation, if caught.

Also, an extraterrestrial, sent to our planet to abduct rubes in the woods, make pretty patterns in cornfields with their UFOs, kill all the men, and make wild nookie with the women. We have only one defense: foil hats for everyone!