What is meant by saturation in the HSB colour model?

In the HSB colour model saturation refers to the perceived difference between one colour and another in terms of vividness.

• • Saturation is measured between a fully saturated colour (100%) and an unsaturated colour that appears dull and washed out until all colour disappears leaving only a monochromatic grey tone (0%).
  • A fully saturated colour is produced by a single wavelength or a narrow band of wavelengths.
  • On HSB colour wheels, saturation is usually shown to increase from the centre to the circumference.