Something that's alien is different or foreign. In science fiction, space aliens come from other planets.

When we talk about aliens, or things that are alien, we're talking about things and people that are different and strange to us — like an alien from outer space. Some people refer to undocumented immigrants as "aliens," but nowadays, the term is often considered dehumanizing. Besides people, experiences can be alien. If you've never owned a cat, then having a cat is alien to you. You can also alienate people by making them feel weird, disconnected, and alone — like an alien.

Definitions of alien
  1. adjective
    being or from or characteristic of another place or part of the world
    alien customs”
    synonyms: exotic
    foreign, strange
    relating to or originating in or characteristic of another place or part of the world
  2. noun
    a person who comes from a foreign country; someone who does not owe allegiance to your country
    synonyms: foreigner, noncitizen, outlander
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    a native or naturalized member of a state or other political community
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    au pair
    a young foreigner who lives with a family in return for doing light housework
    deportee, exile
    a person who is expelled from home or country by authority
    a Latin American (disparaging) term for foreigners (especially Americans and Englishmen)
    import, importee
    an imported person brought from a foreign country
    an alien who paid a fee to reside in an ancient Greek city
    au pair girl
    a foreign girl serving as an au pair
    type of:
    traveler, traveller
    a person who changes location
  3. noun
    anyone who does not belong in the environment in which they are found
    synonyms: stranger, unknown
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    acquaintance, friend
    a person with whom you are acquainted
    foreigner, outsider
    someone who is excluded from or is not a member of a group
    one living on or coming from the other side of the Alps from Italy
    type of:
    interloper, intruder, trespasser
    someone who intrudes on the privacy or property of another without permission
  4. noun
    a form of life assumed to exist outside the Earth or its atmosphere
    synonyms: extraterrestrial, extraterrestrial being
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    type of:
    hypothetical creature
    a creature that has not been observed but is hypothesized to exist
  5. adjective
    not contained in or deriving from the essential nature of something
    “an economic theory alien to the spirit of capitalism”
    synonyms: foreign
    not forming an essential part of a thing or arising or originating from the outside
  6. verb
    transfer property or ownership
    “The will aliened the property to the heirs”
    synonyms: alienate
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    type of:
    cause to change ownership
  7. verb
    arouse hostility or indifference in where there had formerly been love, affection, or friendliness
    synonyms: alienate, disaffect, estrange
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    drift apart, drift away
    lose personal contact over time
    detach the affections of
    type of:
    alter, change, modify
    cause to change; make different; cause a transformation
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