The process by which materials (such as colloids, soluble salts, and mineral particles) are leached out of one layer of soil and are deposited in another. For obvious reasons, this is almost always a higher soil layer being leached into a lower. The resulting deposits are in turn called illuvium.
The word illuviation was formed as a variation on alluviation (from alluvium); with the addition of the Latin illuviēs, meaning 'in that which is washed'.