Source : WordNet®

user interface
     n : (computer science) a program that controls a display for the
         user (usually on a computer monitor) and that allows the
         user to interact with the system [syn: {interface}]

Source : Free On-Line Dictionary of Computing

user interface
     
        The aspects of a computer system or program which can be seen
        (or heard or otherwise perceived) by the human user, and the
        commands and mechanisms the user uses to control its operation
        and input data.
     
        A {graphical user interface} emphasises the use of pictures
        for output and a pointing device such as a {mouse} for input
        and control whereas a {command line interface} requires the
        user to type textual commands and input at a keyboard and
        produces a single stream of text as output.
     
        See also {user interface copyright}.
     
        (1995-02-20)