A demonym is the word used to describe people coming from a certain place. For example, to describe someone from Europe, the demonym “European” is used. For America, it is “American”. Australia is “Australian” and so on.
From these first three extremely common examples, it would seem that a demonym is simply tacking on the letters “an” or “ian” to the end of a word. But this is not the case.
Some countries have “an”,or “ian”. But there’s also “ite”, “er”, “ish” and “ese”, plus many more.
I always wondered what the rule is, as the usage seems to be all over the place.