The Collaborative International Dictionary of English v.0.48:

Glacis \Gla"cis\, n. [F. glacis; -- so named from its
   smoothness. See Glacier.]
   A gentle slope, or a smooth, gently sloping bank; especially
   (Fort.), that slope of earth which inclines from the covered
   way toward the exterior ground or country (see Illust. of
   Ravelin).
   [1913 Webster]

Moby Thesaurus II by Grady Ward, 1.0:

79 Moby Thesaurus words for "glacis":
   abatis, advanced work, balistraria, bank, banquette,
   barbed-wire entanglement, barbican, barricade, barrier, bartizan,
   bastion, battlement, bevel, bezel, breastwork, bulwark, casemate,
   cheval-de-frise, chute, circumvallation, contravallation,
   counterscarp, curtain, demibastion, dike, drawbridge, earthwork,
   easy slope, enclosure, entanglement, escarp, escarpment, fence,
   fieldwork, fleam, fortalice, fortification, gentle slope, grade,
   gradient, hanging gardens, helicline, hillside, inclination,
   incline, inclined plane, launching ramp, loophole, lunette,
   machicolation, mantelet, merlon, mound, outwork, palisade, parados,
   parapet, pitch, portcullis, postern gate, ramp, rampart, ravelin,
   redan, redoubt, sally port, scarp, sconce, shelving beach, side,
   slope, steep slope, stiff climb, stockade, talus, tenaille,
   vallation, vallum, work