The group velocity of a wave is the velocity with which the overall shape of the waves' amplitudes known as the modulation or envelope of the wave propagates through space.
Formula:
vg(ω) = ∂ω / ∂k
where,ω - is the wave's angular frequency (usually expressed in radians per second)
k - is the angular wavenumber (usually expressed in radians per meter)
vg(ω) - group velocity