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]