Nationality: noun or adjective

[Sajan]

"Are you American?"

Why is "American" used here without an article?
Is it an adjective? If yes, then is there any difference btw "an American" and "American" when talking about sb's nationality?

[Mister Micawber]

.
An American-- this is a noun.
American -- this is an adjective.

For some nations, we use the same word for both: American, Canadian, Japanese, Iraqi.
For other nations, we use different words: Swedish, Swede; Briton, British; French, Frenchman.
.
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Native English teacher at Mister Micawber's

[Sajan]

I see, thank you. So when it's about "American", there's no difference in meaning btw a noun and an adjective concerning a person's nationality, right?

[Mister Micawber]

.
Right.
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_________________
Native English teacher at Mister Micawber's

[Sajan]

Thanks. =)