Flutter is a development kit and open-source framework for creating mobile applications for Android and iOS, web applications, as well as desktop applications for Windows, macOS and Linux using the Dart programming language, designed and developed by Google Corporation.

The first version released in 2015 under the name "Sky", worked only for Android applications. The main claimed feature is high graphical performance (the ability to display 120 frames per second). Full support for creating web applications appeared in version 2.0 (March 2021), from the same version support for desktop applications for Windows, macOS and Linux and Google Fuchsia (in the Dart virtual machine with JIT-compiler) was implemented.

Due to restrictions on dynamic code execution in the App Store, under iOS Flutter uses AOT compilation[3]. A widely used feature of the Dart platform is "hot reloading", where source code changes are applied immediately to a running application without the need to restart it.

The main components of the kit are Dart platform, Flutter engine, Foundation library, widget sets and development tools (Flutter DevTools).

Flutter engine is written primarily in C++, supports low-level rendering using the Google Skia graphics library, has the ability to interact with platform-dependent SDKs for Android and iOS.

Foundation library[4], written in Dart language, contains basic classes and methods to create Flutter applications and interact with the Flutter engine.

The user interface design of Flutter applications involves the use of widgets, described as immutable objects of any part of the user interface. All graphical objects, including text, shapes and animations, are created using widgets; combining simple widgets creates complex widgets. The framework comes with two main sets of widgets - Material Design (Google style) and Cupertino (Apple style).