A glacier is a large persistent body of ice that forms where the accumulation of snow exceeds its ablation (melting and sublimation) over many years, often centuries.

Glaciers slowly deform and flow due to stresses induced by their weight, creating crevasses and other distinguishing features. They also abrade rock and debris from their substrate to create landforms such asmoraines.

Glaciers form only on land and are distinct from the much thinner sea ice and lake ice that form on the surface of bodies of water.