Sedimentation is the process, not the stuff. It is what happens when small pieces of rock, earth, or other solids slowly sink and form a layer at the bottom of the sea, a river, a lake, or even a tank of water. Over a very long time, that falling-and-piling can build the layers that later become sedimentary rock. In a lab or a water plant, people also talk about sedimentation in a tank: you wait, the dirt falls, and you take the clearer water off the top. The word sounds scientific and a little formal. Easy way to hold the family: sediment is the pile, sedimentary describes the rock, sedimentation is the sinking itself.