Indexed color image
From The Glossary of Digital Photography
Color space that uses a color lookup table to reduce the sizes of images. Rather than the more typical 3-color-per-pixel representation used in an RGB color space, an indexed color space uses only 8 bits of color information to represent a pixel. These 8 bits refer to a table with all of the colors listed. Because of the fewer possible colors, an indexed color image does not have all of the fine gradations of color. The sizes of images are greatly reduced, but blockiness in smooth tone areas can occur. It is possible to use fewer or more than 8 bits, but 8 bits, which translates to 256 colors, is the usual number of colors possible with an indexed color image. The GIF format, popular for web usage, is an example of an indexed color space. See also Look-up table (LUT)
