dependence of phase velocity on frequency
Dispersion is the idea that the frequency of a wave influences its velocity (speed). Dispersion is most easily seen in the refraction of light, when all the colors inside white light become separated by a prism.
When a white beam is shone at the prism, all the colors in the light (color spectrum) are slowed when they meet the prism. Each color then refracts (bends) at a different angle, making us see a rainbow.
Dispersion is also important in telecommunication when different parts of a signal arrive at different times.