Metro is the name originally given by Microsoft to the user interface design introduced with Windows Phone 7 and Windows 8

Metro is the name of the new design language created for the Windows Phone 7 interface. When given the chance for a fresh start, the Windows Phone design team drew from many sources of inspiration to determine the guiding principles for the next generation phone interface. Sources included Swiss influenced print and packaging with its emphasis on simplicity, way-finding graphics found in transportation hubs and other Microsoft software such as Zune, Office Labs and games with a strong focus on motion and content over chrome.

( http://www.microsoft.com/design/toolbox/tutorials/windows-phone-7/metro/ , retrieved 2013-01-11)

As of August 2012 the term "Metro" is no longer supported by Windows after a legal dispute. (See http://en.wikipedia.org/wiki/Metro_%28design_language%29#Legal_issues, as of 2013-01-11)

Further reading:

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