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]

Moby Thesaurus II by Grady Ward, 1.0:

95 Moby Thesaurus words for "parapet":
   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,
   breastwork, 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, portcullis, postern gate, rampart,
   ravelin, redan, redoubt, roadblock, rock-fill dam, sally port,
   scarp, sconce, seawall, shutter dam, stockade, stone wall,
   tackling, tenaille, vallation, vallum, wall, weir, wicket dam,
   work