The three additive color primaries, or the three basic components of visible light, the various combinations of which produce all the colors of the spectrum. (See Additive Color Primaries.)
In computer graphics, RGB refers to a type of color image display (called an RGB monitor) or color space in which each of those three color components is stored and transmitted to the display separately, the intensity and percentage of each varying by pixel, by an amount determined by the color depth of the monitor.
In printing, red, green, and blue are secondary colors produced by the overprinting of dots of the subtractive color primaries—cyan, magenta, and yellow. (See Subtractive Color Primaries.)