"An Internationalised domain name (IDN) is an Internet domain name that (potentially) contains non-ASCII characters" (such as extended Latin characters, diacritics, or non-Latin scripts).
The Internet Corporation for Assigned Names and Numbers (ICANN) is responsible for regulations that affect use of IDNs.
See also:
ICANN, "Internationalized Domain Names," http://www.icann.org/topics/idn.html
Wikipedia, "Internationalized domain name," http://en.wikipedia.org/wiki/Internationalized_domain_name
