A glacis (/ˈɡleɪ.sɪs/, French: [ɡlasi]) in military engineering is an artificial slope as part of a medieval castle or in early modern fortresses. They may be constructed of earth as a temporary structure or of stone in more permanent structure. More generally, a glacis is any slope, natural or artificial, which fulfils the above requirements. The etymology of this French word suggests a slope made dangerous with ice, hence the relationship with glacier.
A glacis plate is the sloped front-most section of the hull of a tank or other armoured fighting vehicle.