User Interface (UI)
Everything where a user gives input or received output.
The user interface are all elements where a user can receive output from a system or can give input into it.
The user interface are the elements that give the user a chance to interact with the system. Most systems have at least one input option that the user can use to pass information to the system that it should then use to give a response.
The response will be given to the user through an output element of the user interface. That makes it possible for the user to perceive new information that might then lead to another input action through the UI input elements.