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Reference.org
Group velocity
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<p>The group velocity of a <a href="/facts/Wave/fU0HH6St">wave</a> is the <a href="/facts/Velocity/Q3o1LVDe">velocity</a> with which the overall envelope shape of the wave's <a href="/facts/Amplitude/aEckuXJM">amplitudes</a>—known as the <i>modulation</i> or <i><a href="/facts/Envelope_(waves)/3JsLqPoh">envelope</a></i> of the wave—propagates through space. </p><p>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, also known as a <a href="/facts/Capillary_wave/KnCg8GAg">capillary wave</a>. The expanding ring of waves is the wave group or <a href="/facts/Wave_packet/4HLR2joo">wave packet</a>, within which one can discern individual waves that travel faster than the group as a whole. The amplitudes of the individual waves grow as they emerge from the <a href="/facts/Trailing_edge/ymBLLVBl">trailing edge</a> of the group and diminish as they approach the leading edge of the group. </p>