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.
[1] It is a water-assisted transport in a basically vertical direction, as compared to alluviation, the horizontal running water transfer.
[2] Illuvium includes organic matter, silicate clay, and hydrous oxides of iron and aluminum.
When percolating rain water reaches a drier soil horizon, water from the suspension is removed by capillary action of microchannels, leaving fine deposits (cutans) oriented along percolation macrochannels.