Berm
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A berm is a level space or shelf separating two features.
Archeologists use the term to define narrow spaces such as those between banks and ditches . It can also be used to describe a linear bank raised to separate two areas.
In mediaeval military engineering a berm or berme was a level space between a parapet or defensive wall and an adjacent steep-walled ditch or moat. It was intended to reduce soil pressure on the walls of the excavated part, to prevent it collapsing. In the trench warfare of World War I, the name was applied to a similar feature at the lip of a trench which served mainly as an elbow-rest for a rifleman whilst firing.
In modern military engineering, berm has come to refer to the wall or parapet itself, when it is constructed of earth or sod. The term especially refers to a low earthern wall adjacent to a ditch which provided the spoil from which the wall was constructed, typically constructed in a continuous action by a bulldozer or combat engineering vehicle. Walls constructed in this manner are an effective obstacle to vehicles, including most armoured fighting vehicles, but are easily crossed by infantry. Because of the efficiency of construction, such walls can be made hundreds or thousands of kilometres long.