For use as the generic term 'alien' which generally refers to a life-form not of Earth, but it can also be used to describe anything, including objects, from another world or system. Use [aliens-1986] for the sequel to the 1979 film Alien; use [alien-franchise] for that entire franchise.
In science fiction, the term 'alien' generally refers to a life-form or being that is not human or of this planet. Typically, aliens come from a place other than the planet Earth, though occasionally they are from different dimensions. Aliens are a common recurring theme in science fiction.