The velocity with which phase fronts propagate in a medium.
In a vacuum the phase velocity is c = 299’792’458 m/s, independent of the optical frequency, and equals the group velocity. In a medium, the phase velocity is typically smaller by a factor of n, the refractive index, which is frequency-dependent. In the visible spectral region, typical Transparent crystals and glasses have refractive indices between 1.4 and 2.8. Semiconductors normally have somewhat higher values.