Acronym for graphical user interface. The use of pictures rather than just words to represent the input and output of a program. Programs with GUIs run under a windowing system (such as X Windows, Microsoft Windows, Apple Macintosh, and so on). GUI programs display icons, buttons, and so on, in windows on the screen; users control the GUI programs mainly by moving a pointer on the screen (typically controlled by a mouse). Also known as a bitmapped interface.