1. A citizen of a country other than the one he or she is currently living in: The alien who was trying to learn English was told that it was not acceptable to stick out his tongue when trying to pronounce th.
2. Anyone who does not belong to, or does not feel accepted by, a group or society.
3. A being from another planet, or another part of the universe; especially, in works of science fiction.
4. Not contained in or deriving from the essential nature of something.