Answer
.NET Multi-platform App UI (.NET MAUI) is Microsoft's cross-platform framework for building native mobile and desktop apps from a single C# and XAML codebase. It targets Android, iOS, macOS (via Mac Catalyst), and Windows (via WinUI 3) from one project, and is the evolution of Xamarin.Forms.
Real-world example A startup builds one MAUI app and ships it to the App Store, Google Play, and the Microsoft Store, sharing roughly 90% of the code instead of maintaining three native teams.