Saturation

Saturation can be defined as being the intensity and the chroma of a colour, which refers to the dominance of a hue in a colour where on the outer edge of the colour wheel the hue can be considered to be more pure. White, black and grey are used to reduce the colour's intensity and saturation. 

As one goes to the centre of the colour wheel the hue of the selected colour is less dominant meaning that the colours directly on the central axis are considered to be desaturated.

Now the image below shows the opposite of the image above, the fact that the outer colours of the colour wheel have more saturation.