Short for red, green, blue monitor, a monitor that requires separate signals for each of the three colors. This differs from color televisions, for example, which use composite video signals, in which all the colors are mixed together. All color computer monitors are RGB monitors.
An RGB monitor consists of a vacuum tube with three electron guns -- one each for red, green, and blue -- at one end and the screen at the other end. The three electron guns fire electrons at the screen, which contains a phosphorous coating. When the phosphors are excited by the electron beams, they glow. Depending on which beam excites them, they glow either red, green, or blue. Ideally, the three beams should converge for each point on the screen so that each pixel is a combination of the three colors.