Windows Presentation Foundation
Windows Presentation Foundation (WPF) is a free and open-source user interface framework for Windows-based desktop applications. WPF applications are based in .NET, and are primarily developed using C# and XAML. Originally developed by Microsoft, WPF was initially released as part of .NET Framework 3.0 in 2006.
Source: Wikipedia — Windows Presentation Foundation (CC BY-SA 4.0)