Avalanche: A large mass of snow, ice and rock in swift motion down a mountainside or over a precipice.

Cleaver: A high, exposed backbone of rock that divides two glaciers.

Cornice: An overhanging mass of wind-drifted snow, ice or rock that forms a ledge that can break off and cause an avalanche.

Crampon: A spiked traction device that fits on a climbing boot.

Crevasse: A stress crack in a glacier's surface caused by the glacier's slow downhill movement, not to be confused with crevices that form in rocks.

Glacier: A large flowing body of ice that moves slowly down a slope or valley. They exist where, over a period of years, snow remains on the ground in all seasons.

Icefall: Refers both to an area of jagged ice blocks, seracs and crevasses caused by a glacier's movement down a steep incline, and to falling ice, analogous to waterfalls in rivers.

Rime Ice: Freezing, wind-blown moisture that collects against objects and forms opaque ice crystals.

Serac: A large tower of ice usually found in or around icefalls.

Snow Bridge: A thick layer of snow and ice covering a crevasse or seasonal creek that is easily broken through when a person crosses it.

Terminus: The down-valley end of a glacier, sometimes referred to as the glacier snout.

Tree Well: A trench surrounding the base of a tree caused by fallen and windblown snow. In areas of heavy snow, tree wells can be more than a dozen feet deep on the windward side.

Whiteout: A blizzardlike weather condition peculiar to glaciers and high altitudes in which no object casts a shadow, the horizon cannot be seen and only dark objects are discernible. It may or may not be accompanied by snow, wind or daylight and is described as walking inside a cloud.

Source: USGS, 1999, and Mount Rainier: A Climbing Guide, 1999.