Complex sentence: As mentioned above, when the focus in a medium is on refraction rather than absorption i.e. on the real part of the refractive index, it is common to refer to the functional dependence of frequency on wavenumber as the "dispersion relation".
Split into simpler sentences: As mentioned above, when the focus in a medium is on refraction rather than absorption. That is, on the real part of the refractive index, it is common to refer to the functional dependence of frequency on wavenumber as the "dispersion relation".