In the 21st Century, the term alien is used mainly to describe two types of entities; in law, an alien is a foreigner to a country or continent. However, in the collective minds of sci-fi fans, conspiracy theorists, and open minded individuals, an alien is an extra terrestrial creature from outer space.
The word "alien" is derived from the Latin term "alienus" which meant stranger, foreigner or someone not related by blood. In the 1300s the Old French "alien" was used. In the 15th Century "residing in a country not of one's birth" was added to the meaning of alien and by 1670 "wholly different in nature" was also added to the meaning. By 1920, the modern "not of this earth" came to be associated with the term alien.
Author H.G Wells was the first to use aliens in a creative setting in his novel "The War Of The Worlds" published in 1898, in which Earth is invaded by Martians (from Mars, obviously.)
In 1902, the Sci-Fi film a Trip To The Moon depicted moon aliens or 'Selinites'. Since then, aliens have been utilised in a variety of media, including books, movies, video games and television, some notable examples being the Aliens franchise, the long running book/comic/audiobook/television series Doctor Who, or even the classic game Space Invaders.