Source : Webster's Revised Unabridged Dictionary (1913)

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}).