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

`, and the wavelength`

**f**`are related as follows:`

**λ****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

`is the magnetic constant (permeability of free space ... 4π x 10`

**μ**_{0}^{-7}Hm

^{-1}).

See group velocity - there's a difference =)