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For questions about a different and uncharacteristic species or object that has come to the world from another world. NOT for questions about extraterrestrials in their native environment.

The term 'alien' in this tag is not used to describe the exclusion of members of the same species from social circles, as sometimes used in the English language.

Aliens or extraterrestrials (for xenobiological species not originating from Earth) are species that are considered foreign to a celestial body, most commonly planet Earth.

For questions regarding the biology or biochemistry of foreign life, consider using the tag.