The saturation of a colour is a measure of its richness, strength or intensity. You could almost say that it’s the amount of colour in the colour. A saturated colour is strong and vibrant; a desaturated colour is subdued or grey. A colour of any hue will approach a pure grey as its saturation approaches zero. Black, white and grey are completely desaturated. The luminance of a colour may limit its saturation because all very dark colours approximate black and all very light colours approximate white.
Saturation is sometimes called the intensity of a colour, its chromatic intensity, its chroma or its purity.