'''Windows RuntimeUI XAML FrameworkLibrary''' ('''WinUI''' also known as '''Windows UIRuntime LibraryXAML Framework''' or '''WinUIWinRT XAML'''; WinUI2WinRT XAML/WinUI 2 was codenamed "Jupiter",<ref>{{cite web | url=http://blog.jerrynixon.com/2012/06/windows-8-15-more-reasons-why-i-choose.html | title=Windows 8: 15 More Reasons why I choose XAML over HTML5 | author=Jerry Nixon | date=20 June 2012 | work=Microsoft}}</ref><ref>{{cite web | url=http://blog.jerrynixon.com/2013/01/devradio-xaml-and-windows-8-app.html?spref=tw | title=DevRadio: XAML and Windows 8 App Development | author=Jerry Nixon, Tim Heuer | date=24 January 2013 | work=Microsoft}}</ref> also known as UWP XAML or simply XAML) is a [[user interface]] [[API]] that is part of the [[Windows Runtime]] [[programming model]] that forms the backbone of [[Universal Windows Platform apps]] (formerly known as ''[[Metro (design language)|Metro]]-style'' or ''Immersive'') for the [[Windows 8]], [[Windows 8.1]], [[Windows 10]] and [[Windows Phone 8.1]] operating systems. It enables declaring user interfaces using [[Extensible Application Markup Language]] (XAML) technology.
The XAML FrameworkWinUI is one of the multiple UI frameworks provided built-in for the Windows Runtime; the others being [[HTML5]] (e.g., via [[WinJS]]) and [[DirectX]].
WinUI 3 decouples the WinRT XAML framework from the operating system as a separate package to be updated quickly and make new features work on older versions of Windows.<ref>{{Cite web|last=Blog|first=Windows Developer|date=2020-07-07|title=A deep-dive into WinUI 3 in desktop apps|url=https://blogs.windows.com/windowsdeveloper/2020/07/07/a-deep-dive-into-winui-3-in-desktop-apps/|access-date=2021-05-28|website=Windows Developer Blog|language=en-US}}</ref> It is part of Windows App SDK (codenamed "Project Reunion"), a Microsoft effort to reconcile the Windows desktop (Win32) and the UWP low IL app model.
