From AMS Glossary
Light scattered into quasi-horizontal viewing directions by the atmosphere's molecules and small particles (usually excluding fog and raindrops).
On clear to partly cloudy days, airlight is dominated by singly scattered sunlight, but it also includes multiply scattered sunlight from clouds, the surface, and the clear sky. A more restrictive definition limits airlight to clear days; a less restrictive one does not limit the elevation angles at which airlight is seen. As haze concentrations increase, airlight radiances increase near the horizon, although not without limit. Increased airlight reduces a distant object's contrast, thus reducing its visibility. At sufficiently large horizontal optical thicknesses, airlight reduces this contrast below the threshold contrast, and the object is then visually indistinguishable from its surroundings. Airlight and the artistic term aerial perspective are related, since airlight causes the reduced contrast associated with increased distance to an object. Airlight is distinct from airglow that originates in the upper atmosphere.