This page does not exist to tell user what is or is not a Seijin, as the topic allows for a wide range of possibilities, but exists to deliver context to users, particularly new users.
The term 'Seijin' is a fan created term from the names of certain monsters in Tokusatsu monsters/Kaiju. The names of these creatures, when translated often become Alien (Insert name). A fine example Alien Baltan is the english name for Baltan Seijin.
Seijin comes from the words Seikai, meaning world, and Jin/jinbei meaning man/person and people/mankind. Seijin means planet/world person/people, or someone from a planet. So Baltan Seijin means, Baltan planet person/people, or as we would say in English Baltanian (which was used in Ultraman Powered's series).
The word is also be switched with Uchujin, which means Spaceman/people, i.e. someone from space. Or, in layman's terms, aliens.
Note: in your stories, you should never call an alien a Seijin, even if the character speaking is Japanese. Seijin is part of their species name in japanese and therefore grammatically wrong. You the narrator of the story can use it, but not the characters. Uchujin is more acceptable.
Now you are probably wondering why, we don't just use space monster or Uchu Kaiju, that's because Seijin...don't really qualify as Kaiju. Yes, they are strange life forms, to normal Earth Creatures, but calling a Seijin a Kaiju, is like calling humans animals. While technically correct, it is fundamentally wrong.
- Sentience: I mean complete sentience, full thinking, self-aware. Sentience is what seperates a Seijin from a Kaiju. No matter what other traits your alien has, if it isn't sentient, its not a Seijin.
- Alien Powers: Now these 'powers' could simply quirks of biology (the evolved to do something humans cannot like, a long mandible tongue), a result of advance self-engineering, technological in nature, a result of their advance science/life style or a result of the alien's biology reacting with Earth's environment (ala Superman). Now not all aliens have to have 'super powers' some could be weaker than humans, like the Alien Chamuda (the blue skinned men) from Ultraseven X. Their 'power' could simply be that they are smarter than most/any human.
- Over Technology: It is almost expected for alien to have some form of advance tech, even if it is only a space craft.