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 waves.

See phase velocity.