Saturation - Saturation refers to the purity or intensity of a color. A "vivid" color is a highly saturated color and a "muted" or "dull" color is a low saturation color. Saturation generally diminishes when colors are mixed.
The image below illustrates a change in saturation from high saturation blue on the left to low saturation blue on the right. The gray squares are provided for reference as they indicate the brightness of the appropriate blue square. Note that while the saturation changes, the brightness does not.