Sediment is made of small particles deposited in water or on land. After deposition, sediments may stay in their new place a long time and be covered by newer sediments. Pressure may slowly transform sediment into sedimentary rocks.

The word sediment is also used for material transported by wind or water, and deposited on the surface. Sediments are often eroded again and deposited in a new place. The study of how sedimentary rocks are formed is called sedimentology.