According to the wiki page, this diagram: shows the phase velocity and group velocity moving in opposite directions. I think I understand what group velocity is, its the velocity of the overall wavepacket, in the diagram I can see a Gaussian wavepacket moving to the right so I'm guessing thats the group velocity. I can't see the phase velocity. I don't know what I'm looking for because I don't really know what phase means. I understand the concept of constructive and destructive interference and how 2 waves can be in or out of phase but I don't understand what phase means with respect to an individual wave. Does the word phase mean where the waveform is on the y axis, at a particular point on the x axis? Thats the idea I have of phase and with that in mind, I can't understand how phase can have a velocity.