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.^{[source needed]} 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.^{[source needed]}

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