Flutter is Google's open-source UI framework for building natively compiled, cross-platform applications from a single codebase, covering mobile (iOS, Android), web, desktop, and embedded devices. Powered by the Dart programming language, Flutter uses a reactive widget tree where everything—from layout containers to buttons—is a widget, enabling highly composable and declarative UI development. Unlike traditional frameworks that rely on platform-specific UI components, Flutter renders directly to a Skia canvas, delivering pixel-perfect control and 60fps+ performance across all targets. The key insight: widgets are immutable and stateless by default—state changes trigger rebuilds rather than mutations, making UI updates predictable and testable. Mastering Flutter means understanding when to use StatelessWidget vs. StatefulWidget, how the widget tree reconciles during rebuilds, and why BuildContext is your gateway to navigating ancestors (Theme, MediaQuery, InheritedWidget) without prop drilling.