In terms of science fiction, an alien is a life form indiginous to an environment outside the planet Earth. Many are sentient humanoids physically indistinguishable from a standard human being, while others, such as the Xenomorph, bear physiology completely unlike anything found on the planet Earth. Aliens are frequently used as both protagonists and antagonists in the science fiction medium and have practically become a staple for the genre as a whole.
(See Also: Alien; Aliens)
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