The phase velocity is a term describing the velocity of a point of constant
(phase) in a pure sine wave through or across a medium.

The phase speed **v**, the frequency **f**, and
the wavelength **λ** are related as follows:

**v = λf**

The phase speed can vary depending on the medium through which it's travelling,
and what mode of propagation. For example, in electromagnetic waves travelling
through space the phase speed `v` is related as:

`byv`^{2} = 1/ε_{0}μ_{0}

where` `**ε**_{0} is the electric constant (absolute
permittivity of free space ... 8.854 x 10^{-12} Fm^{-1}), and
**μ**_{0} is the magnetic constant (permeability
of free space ... 4π x 10^{-7} Hm^{-1}).

See group velocity - there's a difference =)