Alien

Alien was (or is) a broad, subjective term, which could be applied as both a noun and an adjective, for any entity, object, place or practice which was not familiar. When referring to entities, this could be used to describe both sentient and non-sentient organic creatures, as well as robots. In most occurrences, this meant entities which were not native to Earth and thus were considered "alien" from the viewpoint of the human race. However, to any species not native to Earth, a human would be considered alien.

The Third Doctor referred to the Silurians as alien beings even after he discovered they had ruled the planet Earth millions of years ago. (DW: Doctor Who and the Silurians) The Eleventh Doctor later stated that they were not aliens but rather "Earth...liens". (DW: The Hungry Earth)