In common everyday usage the word "American" in the term "American citizen" always refers the United States of America, the country, and NOT a continent. Anyway, there is no continent called America; there is North America and South America. In my entire life, over here in North America (Canada), the word America always refers to the U.S.
Well, America is a continent, so it should be from the continent. According to Wikipedia " (Conceptually,) citizenship is focused on the internal political life of the state" which means, the term "American citizen" is simply wrong or deceptive or on the best presumptuous, if exclusively claimed by citizens of only one of the many American states.
It's like you would ask a random number of people from Spain, French, Germany, the Netherlands, Sweden etc (which all belong to the European Union); or from (the European part of) Turkey, Swiss, Norway, (the European part of) Russia, Armenia or Island (which are not members of the EU) etc. what their citizenship is. None of all, no matter if from a state inside or outside the EU, would say or claim to be a European citizen because there is only one continent on earth that can offer continental citizenship - Australia, all others can not.
If you search on Startpage, DuckDuckGo, Bing, Unbubble or any search machine for "American citizenship, you will find out, that all governmental US-Administrations show up with the term U.S. Citizen(ship).
In less words: there is not such a thing like American citizen(ship).
Lol! Terms American citizen and US citizen are interchangeable according to US Immigration department.
Thank you for this interesting information. But I have a problem with that link. You claim: "Terms American citizen and US citizen are interchangeable according to US Immigration department"
It seems to me, that this is not a legit governmental website but some kind of a blog from somebody who distributes or sells software and forms. I get a page that states on the very top and in capital letters:
"This website is not affiliated with, owned, or operated by any government agency"
and a bit further down "We are not a law firm, and this site and our software are not a substitute for the advice of a lawyer and do not contain or constitute legal advice......"
Yet your posting triggered my curiosity. If you find any governmental site, that actually states, that the term american citizen is officially acknowledged and used in official forms, than it would be nice if you could share the link.