UI refers to "User Interface" and can be defined as everything designed into an information device with which a person may interact.
These devices can be display screens, keyboards, a mouse or the appearance of a desktop or mobile. UI can also refer to the way in which a user interacts with a website or app.
The growing use of web and mobile apps among consumers, have led companies to place increased importance and priority on UI in an effort to improve a user's overall experience.
In fact, the emerging popularity of mobile apps in particular has affected the evolution of UI - leading to a development called Mobile UI. Mobile UI is specifically concerned with creating usable, interactive interfaces on the smaller screens of smartphones and tablets. It is focussed primarily on improving special features on these devices, like touch controls.
UI is often talked about alongside UX (User Experience), which relates to the aesthetic appearance of a device. UX is also concerned with response time and the content that is presented to the user within the context of UI, or the user interface. The two are inextricably linked and are evolving together.