An alien is a general term for any entity which is not from your native planet. In most occurrences, this means entities which are not native to Earth, from the viewpoint of the Humans. This can include both sentient and non-sentient creatures, as well as most extraterrestrial robots.
- The Doctor once referred to the Silurians as an alien race. This might imply that they are actually not native to Earth, or, more likely, that the Third Doctor was simply uninformed.
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