Webster's Revised Unabridged Dictionary · 1913

Saturation

Saturation , noun

[Latin saturatio: compare French saturation.]

1.
The act of saturating, or the state of being saturating; complete penetration or impregnation.
2.
(Chemistry) The act, process, or result of saturating a substance, or of combining it to its fullest extent.
3.
(Optics) Freedom from mixture or dilution with white; purity; -- said of colors.

The degree of saturation of a color is its relative purity, or freedom from admixture with white.