Chromatic dispersion is the change of index of refraction with wavelength. Generally the index decreases as wavelength increases, blue light traveling more slowly in the material than red light. Dispersion is the phenomenon which gives you the separation of colors in a prism. It also gives the generally undesirable chromatic aberration in lenses. Usually the dispersion of a material is characterized by measuring the index at the blue F line of hydrogen (486.1 nm), the yellow sodium D lines (589.3 nm), and the red hydrogen C line (656.3 nm). The dispersion is measured by a standard parameter known as Abbe's number, or the v value or V number, all of which refer to the same parameter:
Serway & Jewett
Example of Dispersion
Note that a low v value implies high dispersion.