3D UI, or three-dimensional user interface, is a type of interface design that uses depth, lighting, shadows, and realistic objects to create a more interactive and visually appealing experience. Unlike traditional 2D interfaces that appear flat, 3D UI makes buttons, menus, and other elements look more realistic and dynamic. Designers often use 3D UI in websites, mobile apps, games, and digital products to improve user engagement and create modern visual experiences. Tools such as Spline are commonly used to design interactive 3D UI elements and animations.