Extensible Application Markup Language
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XAML (short for eXtensible Application Markup Language, and pronounced "Zammel") is Microsoft's primary interface language for Windows Longhorn. It is a declarative XML-based language.
In typical usage the XML files will be produced by a visual design tool, such as Visual Studio .NET. The resulting XML is compiled rather than interpereted on the fly.
Open Source Equivalents
XUL is seen as an open-source equivalent to 'XAML.