From AMS Glossary
Scattering of radiation, usually electromagnetic but possibly acoustic, by an array of objects (e.g., atoms, molecules, particles) each of which is excited to scatter (radiate) not only by an external source but also by the scattered radiation from the other objects in the array.
Multiple scattering is distinguished from single scattering, an idealization strictly realized only with a single object excited by an infinitely distant source. Scattering as a consequence solely of excitation by the external source is sometimes referred to as primary scattering, the remaining scattering as secondary scattering, which is misleading in that it can be decomposed into an infinite series of primary, secondary, tertiary, and higher-order scattering. Multiple scattering can be classified according to the coherence properties of the array and the external source. For incoherent multiple scattering, phase differences of scattered waves are random, and scattered powers are additive. For coherent multiple scattering, phase differences of scattered waves are not random, and scattered fields are additive. Incoherent and coherent multiple scattering are idealizations. An example of (primarily) incoherent multiple scattering is scattering of sunlight by thick clouds. An example of (mostly) coherent multiple scattering is specular reflection by a glass of water. Scattering by a single cloud droplet is an example of scattering by a coherent array—the water molecules in the droplet stick together (cohere) in the sense that the phase differences between their individual scattered waves are fixed—whereas scattering by the entire cloud is incoherent in the sense that for droplets separated by random distances large compared with the wavelength, the phase differences between waves scattered by individual droplets are essentially random.