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a. one of a pair of wood, metal, or plastic runners that are used for gliding over snow. Skis are commonly attached to shoes for sport, but may also be used as landing gear for aircraft, etc.
b. (as modifier): a ski boot
2. a water-ski
i. A long, narrow runner of wood or metal, upturned at the front and projecting ahead of the landing gear to prevent nosing over. This rigid member used to be fitted in early airplanes.
ii. In helicopters, fixed tubular landing gear. The landing gear may be provided with auxiliary wheels to permit ground mobility. The helicopter, in this case, resorts to hover taxiing for movement on ground.
iii. A type of landing gear to permit aircraft to operate on snow and ice. Installation of skis may or may not involve removal of the wheels. Retractable skis have a slot in their bottom surfaces, so they can be pulled up enough for the wheels to protrude below the skis, enabling the aircraft to land on a hard surface on its wheels.