The group velocity is one version of the speed of light. When a short pulse of light moves through a medium other than vacuum, it can be shown experimentally that the individual crests of the light waves move at a different velocity to the pulse of light as a whole. The velocity of the individual crests is called the [Phase velocity]?. The velocity of the whole pulse is called the Group velocity.
The group velocity is in most cases a very good approximation to the speed that information can be sent through the medium using light.
In certain rare cases, such as Xrays through most solids, and also microwaves through a wave guide, the Phase velocity can be greater than the speed of light in a vacuum, but the Group velocity remains less that the speed of light in a vacuum.
Recently, some bizarre cases have been found where the group velocity is faster than the speed of light in a vacuum. It appears (though this may still be slightly controversial) that in these cases information still travels slower than the speed of light in a vacuum. (If information could travel faster than light in a vacuum there would be interesting consequences: see Theory of relativity.)
Group velocity is mostly already covered under dispersion, this article should probably be a stub with a link to that one. BTW, "Recently, some bizarre cases have been found where the group velocity is faster than the speed of light" is incorrect, these cases were studied and understood by Sommerfield and Brillouin c.1930. What's new is that they used stimulated emission in the medium to amplify the light, that makes it much easier to see the effect. -- DrBob
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