A glacis, in military engineering (see Fortification and Siege) is an artificial slope of earth in the front of works, so constructed as to keep an assailant under the fire of the defenders to the last possible moment. On the natural ground level, troops attacking any high work would be sheltered from its fire when close up to it; the ground therefore is raised to form a glacis, which is swept by the fire of the parapet. More generally, the term is used to denote any slope, natural or artificial, which fulfils the above requirements.

The forward armor on a tank is called the glacis plate, and is usually the thickest and strongest armor on the vehicle.