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:
byv2 = 1/ε0μ0
where ε0 is the electric constant (absolute
permittivity of free space ... 8.854 x 10-12 Fm-1), and
μ0 is the magnetic constant (permeability
of free space ... 4π x 10-7 Hm-1).
See group velocity - there's a difference =)