The Collaborative International Dictionary of English v.0.48:

Parapet \Par"a*pet\, n. [F., fr. It. parapetto, fr. parare to
   ward off, guard (L. parare to prepare, provide) + petto the
   breast, L. pectus. See Parry, and Pectoral.]
   [1913 Webster]
   1. (Arch.) A low wall, especially one serving to protect the
      edge of a platform, roof, bridge, or the like.
      [1913 Webster]

   2. (Fort.) A wall, rampart, or elevation of earth, for
      covering soldiers from an enemy's fire; a breastwork. See
      Illust. of Casemate.
      [1913 Webster]

WordNet (r) 3.0 (2006):

parapet
    n 1: a low wall along the edge of a roof or balcony
    2: fortification consisting of a low wall [syn: parapet,
       breastwork]