Thread:LucidPigeons/@comment-27586321-20151025184242/@comment-27560932-20151025184856

Demons are generally seen as fantastical, evil "spirits" whilst aliens (though the term here is broad enough to include any entity from another dimension or planet) are generally more seen as a sort of "sci-fi" villain. If the demon originates from Hell or a similar stand-in, Hell or the Hell stand-in has to be explicitly shown as a different world or dimension (example being Apokolips, Darkseid's realm). If the demon crosses far out enough of the realm of fantasy and more into the world of sci-fi while still being presented as an alien, they're less of a malevolent spirit and thus cannot be considered demons.