Dispersion
Definition
White light splitting into a rainbow because different colors bend by different amounts in glass or water.
Technical definition
The wavelength dependence of refractive index, n(λ): shorter wavelengths (blue) refract more than longer ones (red), separating white light into its spectrum. A prism's angular spread quantifies a material's dispersive power.
Example
A glass prism paints a rainbow on the wall because blue light bends about 1–2° more than red light through the glass. Real rainbows are the same physics in reverse: sunlight disperses inside millions of raindrops.
Dispersion is not diffraction or scattering. Diffraction spreads waves around obstacles; scattering bounces light off particles (why the sky is blue); dispersion separates colors because the material's refractive index itself varies with wavelength. A prism's rainbow needs no particles and no edges — just glass, and the fact that blue light travels slightly slower in it than red light does.