A glacier is a large mass of ice moving slowly over some land surface or down a valley, formed over long periods from the accumulation of snow in areas where the amount of snow that falls exceeds the amount that melts. The word glacier comes from French via the Vulgar Latin glacia, and ultimately from Latin glacies meaning ice.

Glacier ice is the largest reservoir of fresh water on Earth, and second only to oceans as the largest reservoir of total water. Glaciers cover vast areas of the polar regions and are found in mountain ranges of every continent except Australia, as well as in New Zealand. In the tropics glaciers are restricted to the highest mountains. The processes and landforms caused by glaciers and related to them are referred to as glacial. The process of glacier growth and establishment is called glaciation. Glaciers are sensitive monitors of climate conditions and are crucial to both world water resources and sea level variation.

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