Alien primarily refers to:
Extraterrestrial life, life which does not originate from Earth
Specifically, intelligent extraterrestrial beings; see List of alleged extraterrestrial beings
Alien (law), a person in a country who is not a national of that countryAlien(s), or The Alien(s) may also refer to:
== Science and technology ==
Introduced species, a species not native to its environment
Alien (file converter), a Linux program
AliEn (ALICE Environment), a grid framework
Alien Technology, a manufacturer of RFID technology
== Arts and entertainment ==
Alien (franchise), a media franchise
Alien...