Aliens are foreigners to their surroundings:
In biology the adjective alien, as in alien species, means that the normal local is somewhere else, that a species or specimen exceptionally has been established in the area in question. Words with similar meaning in biology include exotic, adventive, introduced, and naturalized. These terms contrast with terms as indigenous and endemic.
Fiction and conspiracies
In popular fiction and conspiracy theories, alien is a term that is often used to refer to life forms, especially intelligent life forms, that are of extraterrestrial origin, i.e. not coming from the Earth. Prime examples of how aliens are viewed are found in the movies Alien, E.T, Signs, and Independence Day.
A peculiarity of this usage is its ethnocentricity: when humans in fictional accounts accomplish interstellar travel and land on a planet 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.