1. The variation of the complex index of refraction with frequency (or, equivalently, wavelength in free space), sometimes classified as normal (if n increases with increasing frequency) or anomalous (if n decreases with increasing frequency). But there is nothing anomalous about anomalous dispersion: Every material substance exhibits anomalous dispersion at some frequencies. Dispersion is a consequence of the inherent frequency- dependent response of individual atoms and molecules to excitation by a time-harmonic field. By means of dispersion a beam of light composed of many frequencies can be spatially separated (angular dispersion) into its components as, for example, with a prism. Rainbows and halos owe their colors to angular dispersion. 2. The spreading of atmospheric constituents, such as air pollutants. Dispersion can be the result of molecular diffusion, turbulent mixing, and mean wind shear. The displacement or advection of polluted air by the mean wind is usually called transport rather than dispersion. The amount of dispersion is usually described statistically by the standard deviation (σx, σy and σz) of pollutant particle locations (x, y, z) from the pollutant puff center-of- mass for isolated short releases such as explosions, or from the plume centerline for continuous emissions such as from a smokestack. For a plume, the local Cartesian coordinate system can be aligned with the x axis pointing in the mean wind direction at plume centerline height, allowing the crosswind and vertical dispersion to be described by σy and σz, respectively. 3. In statistics, the scattering (or degree thereof) of the values of a frequency distribution from their average.