Parapet, n.
Etymology: [F., fr. It. parapetto, fr. parare to ward off, guard (L. parare to prepare, provide) + petto the breast, L. pectus. See Parry, and Pectoral.]
1. (Arch.)
A low wall, especially one serving to protect the edge of a platform, roof, bridge, or the like.
2. (Fort.)
A wall, rampart, or elevation of earth, for covering soldiers from an enemy's fire; a breastwork. See Illust. of Casemate.
Shades of meaning
Fernald has no entry of its own for parapet; he lists it under barrier.
Where he writes about it:
A parapet is a low or breast-high wall, as about the edge of a roof, terrace, etc., especially, in military use, such a wall for the protection of troops; a rampart is the embankment surrounding a fort, on which the parapet is raised; the word rampart is often used as including the parapet. — from barrier