Touch screen
From Sonic Retro
A touch screen is a input device that allows the user to interact with the computer by touching the display screen. Game consoles which use touch screens include the Game.com and Nintendo DS.
Early screens used beams of infrared light projected across the screen surface. Interrupting the beams generated an electronic signal identifying the location of the screen, which was relayed to software.
Modern touch screens use a thin, durable, transparent plastic sheet overlayed onto the glass screen. The location of a touch is calculated from the capacitance for the X and Y axes, which varies based upon where the sheet is touched.
The HP-150 was among one of the world's earliest commercialized touch screen computers. It actually has not a touch screen in the strict sense, but a 9" Sony CRT surrounded by infrared transmitters and receivers which detect the position of any non-transparent object on the screen.