Volumetric fog
Models fog as a participating medium sampled into a camera-aligned 3D grid, accumulating in-scattered light per cell.
Flat distance fog is just a colour blended in by depth; it can't capture shafts of light slicing through mist or a room full of smoke. Volumetric fog samples the medium into a frustum-aligned grid of froxels, computes how much light each cell scatters toward the camera, then marches front-to-back to accumulate the result. Bart Wronski's Assassin's Creed IV talk (SIGGRAPH 2014) made the froxel approach standard. Unlike volumetric lighting, which is about visible light shafts, fog describes the medium itself — though in modern engines the two usually share the same froxel pipeline.