A glacis ( or /ˈglæ.si/) in military engineering is an artificial slope of earth in the front of works such as fortifications, so constructed as to keep any potential assailant under the fire of the defenders to the last possible moment. On the natural ground level, troops attacking any high work have a degree of shelter from its fire when close up to it; engineers therefore raise such ground to form a glacis, which defenders can sweep with fire from the parapet.

More generally, the term glacis can denote any slope, natural or artificial, which fulfils the above requirements.

The etymology of this French word suggests a slope made dangerous by frozen ice, hence the relationship with glacier.

See also: fortification, siege.

The glacis plate forms the front hull armor of a tank or other armored fighting vehicle. It usually comprises the thickest and strongest armor on the vehicle, often well sloped to increase the possibility of deflecting antitank fire.