Aerial Perspective
The reduction of contrast, saturation and detail with distance, caused by atmospheric scattering.
In depth
Also called atmospheric perspective, it makes distant objects appear paler, bluer and softer than near ones, which the eye reads directly as distance. It is the cue that makes a range of overlapping hills legible as receding layers, and removing it too aggressively with a dehaze slider flattens the picture's depth.
Examples in practice
Successive mountain ridges fade from dark green to pale blue-grey, each step reading as further away.