A revetment in stream restoration, river engineering or coastal engineering is a facing of impact-resistant material (such as stone, concrete, sandbags, or wooden piles) applied to a bank or wall in order to absorb the energy of incoming water and protect it from erosion.  River or coastal revetments are usually built to preserve the existing uses of the shoreline and to protect the slope.  
In architecture generally, it means a retaining wall. In military engineering it is a structure formed to secure an area from artillery, bombing, or stored explosives.