Flutter is an open-source UI software development kit created by Google. It is used to develop applications for Android, iOS, Linux, Mac, Windows, Google Fuchsia, and the web from a single codebase. Flutter uses the Dart programming language and follows a reactive programming model, enabling developers to build fast, expressive, and flexible user interfaces with a unified codebase. Flutter's layered architecture allows for customizing every aspect of the UI, from layout and animation to gestures and accessibility. It provides a rich set of pre-built widgets for building common UI elements, as well as support for custom widgets and animations. Flutter's hot reload feature allows developers to quickly iterate on their code and see changes instantly on emulators, simulators, and physical devices. Flutter is used by developers and organizations to build high-quality native and cross-platform mobile applications with a focus on performance, productivity, and user experience.