Alien

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

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