alien

UK: /ˈeɪliən/ US: /ˈeɪljən, ˈeɪliən/ ,(āl′yən, ā′lē ən)

WordReference Random House Learner's Dictionary of American English © 2025

al•ien /ˈeɪlyən, ˈeɪliən/   n. [countable]

1. Government a foreign-born person who is living in a country and has not been legally naturalized or been granted citizenship.
2. a person who has been estranged or excluded;
   outsider.
3. a being or creature from outer space;
   an extraterrestrial.

adj. 

4. Government [before a noun] owing allegiance to another country;
   not naturalized;
   of or relating to aliens.
5. Government foreign:
   [only before a noun] a people accustomed to alien rulers.
6. unlike one's own;
   strange:
   frightened by the alien environment; alien ideas.

See -ali-.

WordReference Random House Unabridged Dictionary of American English © 2025

al•ien  (āl′yən, ā′lē ən), n. 

1. Government a resident born in or belonging to another country who has not acquired citizenship by naturalization (distinguished from citizen).
2. a foreigner.
3. a person who has been estranged or excluded.
4. a creature from outer space;
   extraterrestrial.

adj. 

5. Government residing under a government or in a country other than that of one's birth without having or obtaining the status of citizenship there.
6. Government belonging or relating to aliens:
   alien property.
7. unlike one's own;
   strange;
   not belonging to one:
   alien speech.
8. adverse;
   hostile;
   opposed (usually fol. by to or from):
   ideas alien to modern thinking.
9. extraterrestrial.

• Latin aliēnus, equivalent. to *alies- (ali-, base of alius other + -es- noun, nominal suffix) + -nus adjective, adjectival suffix
• Middle French
• Middle English 1300–50

Synonyms

• 1. immigrant.
• 2. See stranger. 
• 3. outcast.
• 7. exotic, foreign.

Collins Concise English Dictionary © HarperCollins Publishers::

alien / ˈeɪljən ˈeɪlɪən/ noun

1. a person owing allegiance to a country other than that in which he or she lives; foreigner
2. any being or thing foreign to the environment in which it now exists
3. (in science fiction) a being from another world, sometimes specifically an extraterrestrial

adjective

1. unnaturalized; foreign
2. having foreign allegiance: alien territory
3. unfamiliar; strange: an alien quality in a work of art
4. (postpositive) and followed by to: repugnant or opposed (to): war is alien to his philosophy
5. (in science fiction) of or from another world

Etymology: 14th Century: from Latin aliēnus foreign, from alius other