Alien
Alien is a broad, subjective term, which can be applied as both a noun and an adjective, for any entity, object, place or practice which is not familiar. When referring to entities, this can be used to describe both sentient and non-sentient organic creatures, as well as robots. In most occurrences, this means entities which are not native to Earth and thus are considered "alien" from the viewpoint of the human race. However, to any species not native to Earth, a human would be considered alien.
Notes
The Third Doctor referred to the Silurians as alien beings even after he discovered they had ruled the planet Earth millions of years ago. This might imply that they are actually not native to Earth (which is unlikely, as their native planet is often said to be Earth) or what the Doctor was saying was that they were alien to the Brigadier, who had just ordered their destruction with no reason other than his mistrust of them. (DW: Doctor Who and the Silurians)
See Also
