alien

American  

[eyl-yuhn, ey-lee-uhn] / ˈeɪl yən, ˈeɪ li ən /

noun

1. a creature from outer space; extraterrestrial.

2. Often Disparaging and Offensive. a resident of one country who was born in or owes allegiance to another country and has not acquired citizenship by naturalization in the country of residence (distinguished from citizen).

3. a foreigner.

4. a person who has been estranged or excluded.

5. a plant or animal species not originating where it is found but introduced, sometimes accidentally, from its native environment elsewhere.

   Imported plants, seeds, or bulbs may harbor unwanted aliens such as foreign insects or the seeds of other plants.

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adjective

1. unlike one's own; strange; not belonging to one.

   In English class, our teacher treated all slang as alien speech.

   Synonyms:

   nonnative, incompatible, inimical, averse, antagonistic, foreign, unknown, unfamiliar, exotic

2. residing under a government or in a country other than that of one's birth without having or obtaining the status of citizenship there.

3. belonging or relating to noncitizens.

   Under certain conditions, seizure of alien property is authorized.

4. opposed; adverse; hostile (usually followed by to orfrom ).

   Their manifesto is full of ideas alien to modern thinking.

5. extraterrestrial.

6. (of a plant or animal species) not native; introduced from a different environment.

   The bullfrog is native to some Canadian provinces, but is an alien species in British Columbia.

alien British  

/ ˈeɪljən, ˈeɪlɪən, ˈeɪljənɪdʒ, ˈeɪlɪə- /

noun

1. a person owing allegiance to a country other than that in which he 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

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adjective

1. unnaturalized; foreign

2. having foreign allegiance

   alien territory

3. unfamiliar; strange

   an alien quality in a work of art

4. repugnant or opposed (to)

   war is alien to his philosophy

5. (in science fiction) of or from another world

verb

1. rare (tr) to transfer (property, etc) to another

alien Scientific  

/ ā′lē-ən /

1. Introduced to a region deliberately or accidentally by humans. Starlings, German cockroaches, and dandelions are species that are alien to North America but have become widely naturalized in the continent.

2. Compare endemic indigenous

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Sensitive Note

See illegal alien.

Usage

What does alien mean? Historically, the word alien has been used in a way that is now often considered disparaging and offensive: to refer to a person who was born in one country and now lives in another country without having become a citizen. Alien and its more general synonym foreigner are now often considered disparaging and offensive due to being used in a way that’s intended to otherize a person—mark them as different and unwelcome. Such use is typically attributed to xenophobia, racism, and anti-immigrant stances. This sense of alien is found in legal terms like ​​resident alien and illegal alien. General and legal use of these terms has decreased in recent years, but they are still in use in many cases. Relatedly, many object to the word illegal in illegal alien and illegal immigrant due to its implication that a person’s very existence can be illegal. There are other common uses of the word alien that are not considered offensive, but that are also based on the idea of something being unfamiliar or from an entirely different place. Alien can be used as an adjective to describe something as entirely unfamiliar, as in It was the first time he had returned in many years, and the place now seemed alien to him. The word foreign can be used in the same way. The word alien is also commonly used as a noun to refer to a being from another planet, sometimes called an extraterrestrial or a space alien. The classic image of an alien from science fiction and pop culture (the one that comes out of a flying saucer) is a little green humanoid with a big head and large eyes. But aliens have been depicted in many different ways.

Other Word Forms

• alienage
  noun
• nonalien
  noun
• proalien
  adjective

Etymology

Origin of alien

First recorded in 1300–50; Middle English, from Middle French, from Latin aliēnus “belonging to or affecting someone else; someone else's; not one's own; foreign,” equivalent to ali(us) “other” + -ēnus adjective suffix

Explanation

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.

Example Sentences

He cites the 2024 finding on Mars by the Perseverance rover of “a biosignature of heavy-metal reduction by alien microbes, possibly the most incredible discovery of the century.”

From The Wall Street Journal • May 15, 2026

The first song they made was “Tonka 2,” which sounds like a flying saucer beaming up an alien in sonic form.

From Los Angeles Times • May 11, 2026

The object, known as 3I/ATLAS, is only the third confirmed interstellar visitor ever detected, and scientists are now uncovering clues about the alien environment where it formed.

From Science Daily • May 8, 2026

And Moss-Bachrach brings a menacing edge of dissociated weirdness to Sal, the character John Cazale played as a neighborhood space alien.

From Los Angeles Times • Apr. 29, 2026

You’d think I’d just confessed to being an alien her beloved daughter got swapped out with in her sleep.

From "Fast Pitch" by Nic Stone

Related Words

• foreigner
• immigrant
• migrant
• outsider
• refugee
• settler
• stranger
• visitor