Alternatively referred to as a human-computer interface, a UI (user interface) is a description of any visible interface accessible by the user. For example, a UI can describe an operating system GUI (graphical user interface) that allows you to interact with the files on the computer.
Another example of a UI is the interface that allows you to interact with players, characters, and other objects in a game. A game UI can include a health bar or mini-map. A hardware device like a printer can also have a UI that allows the user to interact with the printer settings.