The Collaborative International Dictionary of English v.0.48:

Earthwork \Earth"work`\, n.
   1. (Mil.) Any construction, whether a temporary breastwork or
      permanent fortification, for attack or defense, the
      material of which is chiefly earth.
      [1913 Webster]

   2. (Engin.)
      (a) The operation connected with excavations and
          embankments of earth in preparing foundations of
          buildings, in constructing canals, railroads, etc.
      (b) An embankment or construction made of earth.
          [1913 Webster]

WordNet (r) 3.0 (2006):

earthwork
    n 1: an earthen rampart