From The Collaborative International Dictionary of English v.0.48:
Saturation \Sat`u*ra"tion\, n. [L. saturatio: cf. F.
saturation.]
1. The act of saturating, or the state of being saturating;
complete penetration or impregnation.
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2. (Chem.) The act, process, or result of saturating a
substance, or of combining it to its fullest extent.
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3. (Optics) Freedom from mixture or dilution with white;
purity; -- said of colors.
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Note: The degree of saturation of a color is its relative
purity, or freedom from admixture with white.
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