Wiktionary
n. (context physics English) the propagation velocity of the envelope of a modulated travelling wave, which is considered as the propagation velocity of information or energy contained in it.
Wikipedia
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.
For example, if a stone is thrown into the middle of a very still pond, a circular pattern of waves with a quiescent center appears in the water. The expanding ring of waves is the wave group, within which one can discern individual wavelets of differing wavelengths traveling at different speeds. The longer waves travel faster than the group as a whole, but their amplitudes diminish as they approach the leading edge. The shorter waves travel more slowly, and their amplitudes diminish as they emerge from the trailing boundary of the group.
Usage examples of "group velocity".
And the other cars were keeping pace or pulling ahead, so that the group velocity was deceptive.
In fact, the group velocity for water waves is exactly half the phase velocity.
And the other cars were keeping pace or pulling ahead, so that their group velocity was deceptive.