Flutter is an open-source UI software development kit created by Google, designed for building natively compiled applications for mobile, web, and desktop from a single codebase. It enables developers to create visually attractive, fast, and consistent applications using a rich set of fully customizable widgets. Released initially in 2017, Flutter has gained popularity due to its ability to expedite the development process by allowing for simultaneous output on multiple platforms. The framework uses the Dart programming language and includes a hot reload feature that aids developers in quickly iterating on projects.