A real-life visual effect of very distant objects, often mountains or skylines, appearing muted and indistinct due to the scattering of sunlight by air. Sometimes incorporated into drawn/painted art. May be evidence for an amazing_background (or certainly a detailed one).

Cue Wikipedia :
"As the distance between an object and a viewer increases, the contrast between the object and its background decreases, and the contrast of any markings or details within the object also decreases. The colours of the object also become less saturated and shift towards the background color, which is usually blue, but under some conditions may be some other color (for example, at sunrise or sunset distant colors may shift towards red)."