What is Graphical User Interface?

Legal Definition
The graphical user interface (GUI /ɡ/), is a type of user interface that allows users to interact with electronic devices through graphical icons and visual indicators such as secondary notation, instead of text-based user interfaces, typed command labels or text navigation. GUIs were introduced in reaction to the perceived steep learning curve of command-line interfaces (CLIs), which require commands to be typed on a computer keyboard.

The actions in a GUI are usually performed through direct manipulation of the graphical elements. Beyond computers, GUIs are used in many handheld mobile devices such as MP3 players, portable media players, gaming devices, smartphones and smaller household, office and industrial controls. The term GUI tends not to be applied to other lower-display resolution types of interfaces, such as video games (where heads-up display (HUD) is preferred), or not including flat screens, like volumetric displays because the term is restricted to the scope of two-dimensional display screens able to describe generic information, in the tradition of the computer science research at the Xerox Palo Alto Research Center (PARC).
-- Wikipedia
Legal Definition
Interface, point-of-contact software managing what passes forth and back between the computer and its user. Whatever displays, whatever a user enters, by keystroke, mouseclick, finger or stylus touch, or speech, is managed and controlled by the GUI. Xerox pioneered it and Apple deployed it in its computers. GUI is now used in every operating system by most application programs. The GUI uses graphic elements like dialog boxes, icons, menus, scroll bars, more so than text characters to let the user enter commands and do on-screen manipulations. A GUI is built to use a consistent set of graphical elements. Once the user learns a particular interface, similar programs become usable without additional learning.