Flutter is a mobile app development framework that uses a single codebase to build high-performance, visually appealing, and feature-rich apps for both Android and iOS platforms. Developed by Google, Flutter enables developers to create beautiful UIs, animations, and customized widgets using the Dart programming language. It offers fast development cycles, hot reload, and a wide range of pre-built widgets and tools, making it a popular choice for building cross-platform mobile apps. Flutter also provides native performance, seamless integration, and an excellent development experience for creating top-notch mobile apps.