What is the correct way to define the phenomenon of light dispersion: as the dependence of the speed of light in matter

What is the correct way to define the phenomenon of light dispersion: as the dependence of the speed of light in matter on frequency, or as the dependence of the speed of light in matter on wavelength? Or are both definitions equivalent?

Both of these definitions are equivalent, since the wavelength is proportional to the speed of light in the medium.

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