Posterization

Normally, a grayscale image has 256 level of gray. By thresholding one can reduce this number. Then the gradual change of gray from white to black disappears and the image starts to look like an old-fashioned poster. In this sense, binarization (black and white only) is a special kind of posterization. Color images are posterized in the same way.

Below: the original, posterization, binarization.