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 menus, icons, text entry boxes, etc.
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.