Illuvium is material displaced across a soil profile, from one layer to another one, by the action of rainwater. The removal of material from a soil layer is called eluviation. The transport of the material may be either mechanical or chemical. The process of deposition of illuvium is termed illuviation. It is a water-assisted transport in a basically vertical direction, as compared to alluviation, the horizontal running water transfer. The resulting deposits are called illuvial deposits. Cutans are a type of illuvial deposit.

Illuvium includes organic matter, silicate clay, and hydrous oxides of iron and aluminum.

Illuvial deposits of clays, oxides, and organics accumulate in subsoil as distinctive soil horizons classified as "B horizons" or "zones of illuviation".