The Collaborative International Dictionary of English v.0.48:

Embankment \Em*bank"ment\, n.
   1. The act of surrounding or defending with a bank.
      [1913 Webster]

   2. A structure of earth, gravel, etc., raised to prevent
      water from overflowing a level tract of country, to retain
      water in a reservoir, or to carry a roadway, etc.
      [1913 Webster]

WordNet (r) 3.0 (2006):

embankment
    n 1: a long artificial mound of stone or earth; built to hold
         back water or to support a road or as protection