Responsive app design refers to the process of designing and developing mobile applications that can adapt to different screen sizes and resolutions of various devices, such as smartphones, tablets, and desktops. The design aims to provide an optimal user experience by adjusting the layout, font size, and other interface elements of the app to fit the screen size of the device.

Responsive app design ensures that the app's content and features are easily accessible and usable on any device, without the need for zooming or scrolling excessively. This approach helps to create a seamless and consistent user experience across multiple devices, which is essential in today's mobile-driven world.

To achieve responsive app design, developers use techniques such as fluid grid systems, flexible images, and media queries to create layouts that adapt to the user's device. The design is typically created using a mobile-first approach, where the app is designed first for the smallest screen size and then adapted to larger screens.