The technical features of a device that the user interacts with directly.
Broadly, this can include hardware such as the display, buttons, other controls, and mechanical features.
More commonly, the UI refers to the software-controlled visual elements displayed on the screen that allow the user to interact with the device. This includes on-screen buttons, menus, icons, text entry boxes, etc. It's everything you see on the screen, and everything that you interact with on a device.
Some UIs are easier to use than others (more user-friendly), and some are more customizable than others.
A standardized, consistent UI is a key component of major smartphone platforms such as Windows Mobile and Series 60.