Any domain that contains a letter in a language-specific alphabet other than English, such as Chinese, Arabic, Japanese, Russian, Tamil, Chinese or a Latin alphabet using a character with an accents, such as Spanish, Danish or French. These are stored in the Domain Name System as ascii strings using an endcoding mechanism called Punycode.

So the domain fênêtre.com translates to xn--fentre-kva.com in punnycode.

All encoded domains form an ascii string starting xn-- but when displayed in a browser, display in their native language alphabet.