Light of different colors travels at different phase velocity, so it can spread out physically as it moves through a material - as in the case of a prism. It is a mechanism which causes mixed color light to spread out as the material responds differently with wavelength.
When considering light in a fiber, the different wavelengths will arrive at the output at slightly different times, this is known as temporal spreading. This effect is associated with pulse-spreading, where a pulse gets wider as the shortest and longest wavelengths making up the pulse tend to separate.
Related Terms: Chromatic Dispersion, Group Delay Dispersion (GDD), Material Dispersion