XAML = Extensible Application Markup Language
Extensible Application Markup Language is a declarative XML-based language created by Microsoft that is used for initializing structured values and objects. It is available under Microsoft's Open Specification Promise. The acronym originally stood for Extensible Avalon Markup Language - Avalon being the code-name for Windows Presentation Foundation (WPF).
Basically XAML is a way of defining the appearance of screens/animations etc that Microsoft introduced for use with Silverlight / Expression Blend / WPF applications.
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