The user interface
is the part of a system
exposed to a user. The system can be any kind of machine, also a computerized system or a software application
. As for software, the user interface is usually the only part a user ever sees. The user interacts with a system through its user interface.
The design of the user interface is relevant for the user's understanding (also called the mental model) of the system and thus for the system's usability or user-friendliness.