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 =)