XAP - Silverlight Application Package
This is an application package initially created for Microsoft Silverlight which is a web browser plug-in to enable enhanced media content. It is also currently used for Windows Phone 7 mobile applications. It includes an XAML file called AppManifest.xaml and may include multiple DLL files referenced by the application. It is stored in a ZIP format and is most often used now to distribute most Windows Phone 7 applications.