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Ice screw: A protection device for ice climbing. Looks like a large bolt that can be screwed in hard ice.

Index line: the point at which the direction-of-travel arrow meets the housing on a compass and where the degree reading should be read to establish a bearing.

Insulation: A material used to reduce heat loss. Typical insulative materials found in mountaineering products are down, synthetic wadding, foam and gels (see also Fill).

Internal-Frame Backpack: A backpack designed with its frame inside the pack.