Alien, from the Latin alia meaning "other," is used in English both as a noun and an adjective. As a noun, it refers to a person, animal, plant or other thing which is from outside the land, area or territory under consideration. A person from Mexico would be an alien in the United States. Trout stocked in streams and lakes to which they are not native would be aliens there. The usage as an adjective refers both to the fact or quality of being an alien, and to unfamiliarity in general. Of an incorrigible criminal, one might say that the concept of ethics was alien to him.
In popular fiction, "alien" is a term that is often used to refer to life forms that are of extraterrestrial origin, i.e. not coming from the Earth, and especially for intelligent life forms. A peculiarity of this usage is its ethnocentricity, as follows: When humans written of in fictional accounts accomplish interstellar travel and make planetfall elsewhere in the universe, the local inhabitants of these other planets are usually still referred to as "alien," even though they are the native life form, and the humans are the intruders. This may be seen as a reversion to the classic meaning of "alien" as referring to "other," in contrast to "us" in the context of the writer's frame of reference. See extraterrestrial life.
Directed by Ridley Scott
The crew of the transport ship Nostromo receives a signal which might be of nonhuman origin, they land on a deserted planet and find an ancient ship and the aliens which killed its crew.
The eponymous alien creature is a lethal predator with consistently-surprising abilities and physical forms, and which apparently reproduces by parasitizing living victims.
Directed by James Cameron
Ripley (played by Sigourney Weaver), the only suvivor of the Nostromo, is rescued from space and drafted to assist a squad of Marines investigating an outpost overrun which may have been overrun by the aliens. This film is considered interesting by many critics for the feminist sub-texts which run through it.
Directed by David Fincher
Ripley, again the only survivor, strands on a prison planet and this time brings the alien with her. There are no weapons on the planet, but they have to hunt it down. At the end of the movie, she commits suicide when she learns that she's pregnant with an alien queen.
Directed by Jean-Pierre Jeunet
200 years later, ruthless scientists clone Ripley and the alien from blood specimens taken on the prison planet. The aliens get out of control and Ripley, who now has some alien DNA because the cloning didn't work perfectly tries to escape with a band of smugglers before the ship collides with earth.