Colluvium or hillwash is the name for loose bodies of sediment that has been deposited or built up at the bottom of a low grade slope or against a barrier on that slope, as a result of rainwater or downhill creep. The deposits that collect at the foot of a steep slope or cliff are also known by the same name. Coarse deposits due to rockfall at a cliff base are called talus and if lithified are talus breccias.

Colluvium normally forms fan or wedge-shaped deposits that seal former ground surfaces, making it an important phenomenon in the field of archaeology. Ancient sites can be preserved beneath colluvium if later changes in the landscape such as deforestation encourage a downward movement of material. This build-up process is called colluviation.