The mobile app development framework that allows for cross-platform development using a single codebase is Flutter. Developed by Google, Flutter enables developers to create natively compiled applications for both iOS and Android from a single codebase in the Dart programming language. Its extensive widget library and customizable UI components allow for a cohesive and visually appealing user experience across platforms. The framework’s fast development cycle, along with features like hot reload, significantly enhances productivity and reduces time-to-market for mobile applications.