A glacier (pronounced UK: /ˈɡlæsiə/ glass-ee-ər or US: /ˈɡleɪʃər/ glay-shər) 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. At least 0.1 km² in area and 50 m thick, but often much larger, a glacier slowly deforms and flows due to stresses induced by its weight. Crevasses, seracs, and other distinguishing features of a glacier are due to its flow. Another consequence of glacier flow is the transport of rock and debris abraded from its substrate and resultant landforms like cirques and moraines. Glaciers form on land, often elevated, and are distinct from the much thinner sea ice and lake ice that form on the surface of bodies of water.
The word glacier comes from French. It is derived from the Vulgar Latin glacia and ultimately from Latin glacies meaning ice. The processes and features caused by glaciers and related to them are referred to as glacial. The process of glacier establishment, growth and flow is called glaciation. The corresponding area of study is called glaciology. Glaciers are important components of the global cryosphere.
On Earth, 99% of glacial ice is contained within vast ice sheets in the polar regions, but glaciers may be found in mountain ranges of every continent except Australia, and on a few high-latitude oceanic islands. In the tropics, glaciers occur only on high mountains.
Glacial ice is the largest reservoir of freshwater on Earth, supporting one third of the world's population. Many glaciers store water during one season and release it later asmeltwater, a water source that is especially important for plants, animals and human uses when other sources may be scant.
Because glacial mass is affected by long-term climate changes, e.g., precipitation, mean temperature, and cloud cover, glacial mass changes are considered among the most sensitive indicators of climate change and are a major source of variations in sea level.