The Collaborative International Dictionary of English v.0.48:

Breastwork \Breast"work`\ (br[e^]st"w[^u]rk`), n.
   1. (Fort.) A defensive work of moderate height, hastily
      thrown up, of earth or other material.
      [1913 Webster]

   2. (Naut.) A railing on the quarter-deck and forecastle.
      [1913 Webster]

WordNet (r) 3.0 (2006):

breastwork
    n 1: fortification consisting of a low wall [syn: parapet,
         breastwork]

Moby Thesaurus II by Grady Ward, 1.0:

94 Moby Thesaurus words for "breastwork":
   abatis, advanced work, arch dam, backstop, balistraria,
   bamboo curtain, bank, banquette, bar, barbed-wire entanglement,
   barbican, barrage, barricade, barrier, bartizan, bastion,
   battlement, bear-trap dam, beaver dam, boom, breakwater,
   brick wall, buffer, bulkhead, bulwark, casemate, cheval-de-frise,
   circumvallation, cofferdam, contravallation, counterscarp, curtain,
   dam, defense, demibastion, dike, ditch, drawbridge, earthwork,
   embankment, enclosure, entanglement, escarp, escarpment, fence,
   fieldwork, fortalice, fortification, gate, glacis, gravity dam,
   groin, hydraulic-fill dam, iron curtain, jam, jetty, leaping weir,
   levee, logjam, loophole, lunette, machicolation, mantelet, merlon,
   milldam, moat, mole, mound, outwork, palisade, parados, parapet,
   portcullis, postern gate, rampart, ravelin, redan, redoubt,
   roadblock, rock-fill dam, sally port, scarp, sconce, seawall,
   shutter dam, stockade, stone wall, tenaille, vallation, vallum,
   wall, weir, wicket dam, work