Source : Free On-Line Dictionary of Computing

touch screen
     
         An input device that allows user to interact with
        computer by touching the display screen.
     
        Often this uses beams of infrared light that are projected
        across the screen surface.  Interrupting the beams generates
        an electronic signal identifying the location of the screen.
        Software interprets the signal and performs the required
        operation.
     
        (1995-04-13)