The group velocity is a term describing the velocity at which a pulse consisting
of multiple frequencies moves through or along a medium as a whole. Each frequency
has its own phase velocity, but the superposition of all these frequencies travels
at a group velocity.
Group velocity u is related to phase velocity c of waves
of length l as follows:
u(ω) = c(ω) + l(ω) (dc / dl)
As you can see, the phase velocity c is equal to the group velocity
only when dc/dl == 0, that is, only in the case of nondispersive
See phase velocity.