Updated: August 03, 2023

An extraterrestrial or alien is any extraterrestrial lifeform; a lifeform that did not originate on Earth. The word extraterrestrial means "Outside Earth". The first published use of extraterrestrial as a noun occurred in 1956, during the Golden Age of Science Fiction. Extraterrestrials are a common theme in modern science-fiction, and also appeared in much earlier works such as the second-century parody True History by Lucian of Samosata.

Science fiction aliens are both metaphors and real possibilities. One can probe the nature of humanity with aliens that by contrast illustrate and comment upon human nature. Still, as evidenced by widespread belief in alien visitors ( See UFOs ) and efforts to detect extraterrestrial radio signals, humans also crave companionship in a vast, cold universe and aliens may represent hopeful, compensatory images of the strange friends we have been unable to find. Thus, aliens will likely remain a central theme in science fiction until we actually encounter them.