UI (User Interface) - definition
User Interface is the software front for interacting with the technical features of a mobile phone.
Although the term can also be used for hardware input such as controls or keys, in the area of mobile phones it's most frequently used to refer to the software-controlled elements displayed on the screen that are used to interact with the device. That includes icons in the menus, text boxes, etc.
User Interfaces that are easier to use than others are referred to as more user-friendly.