bit depth

(1) May refer to any coding system that uses numeric values to represent something. The depth, or number of bits, determines how many discrete items can be represented.

(2) The number of bits used to hold a pixel. Also called "color depth" and "pixel depth," the bit depth determines the maximum number of colors that can be displayed at one time. True Color (16M colors) is required for photorealistic images and video. Most display adapters today support 65K and 16M colors at their highest resolution without noticeable loss of performance in rendering the images.

    Bit depth
    Pixel depth
    Color depth   Number of colors

     4-bits       16 (Standard VGA)
     8-bits       256 (Super VGA, indexed color)
    16-bits       65,536 (High Color)
    24-bits       16,777,216 (True Color)
    32-bits       16,777,216 + alpha channel

    15-bits       32,768 (Custom option sometimes
                          available on earlier
                          display adapters)