escarpment

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A steep slope or long cliff that results from erosion or faulting and separates two relatively level areas of differing elevations.
• noun A steep slope in front of a fortification.

from The Century Dictionary.

• noun In fortification, ground cut away, nearly vertically, about a position in order to render it inaccessible to an enemy.
• noun Hence The precipitous side of any hill or rock; the abrupt face of a high ridge of land; a cliff.

from the GNU version of the Collaborative International Dictionary of English.

• noun A steep descent or declivity; steep face or edge of a ridge; ground about a fortified place, cut away nearly vertically to prevent hostile approach. See scarp.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A steep descent or declivity; steep face or edge of a ridge; ground about a fortified place, cut away nearly vertically to prevent hostile approach.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a steep artificial slope in front of a fortification
• noun a long steep slope or cliff at the edge of a plateau or ridge; usually formed by erosion