A ski is narrow strip of wood, plastic, metal or combination thereof worn underfoot to glide over snow. Substantially longer than wide and characteristically employed in pairs, skis are attached to boots with bindings, either with a free, lockable, or permanently secured heel.

Originally intended as an aid to travel over snow they are now mainly used recreationally in the sport of skiing.

Skis are also fitted to vehicles dedidcated to traveling over snow such as snowmobiles and snowcats.

The Nordic ski technology was adapted during the early twentieth century to enable skiers to turn at higher speeds. New ski and binding designs, coupled with the introduction of ski lifts and snowcats to carry skiers up mountains, enabled the development of alpine skis. Meanwhile advances in technology in the Nordic camp allowed for the development of special skis for skating and ski jumping.

Ski nomenclature is relatively simple. Examining the ski from front to back along the direction of travel, the front of the ski (typically pointed or rounded) is the "tip", the middle is the "waist" and the rear (typically flat) is the "tail".

Tanner Hall (born October 26, 1983), of Kalispell, Montana is a freeskier.

Chris Hall's brother. In 2002 Tanner Hall co-founded the freeride ski company Armada Skis with skier JP Auclair and photographer Chris O'Connell.^{[citation needed]} Hall is also part owner of the snowcat ski hill Retallack near Nelson, BC, Canada, along with skier Seth Morrison.^{[citation needed]}

In March 2005 while attempting a switch 900 over the legendary Chad's Gap in Wasatch backcountry, Utah, he pulled up short, hit the knuckle and then ragdolled down the landing, breaking both ankles. His injuries put him out for the rest of the season but returned in 2006 to win gold at both the US Freeskiing Open and the Winter X-Games.^{[citation needed]}

After overshooting a jump at Stevens Pass, in May 2009, in Washington, he has been on hiatus for just over a year. He suffered from tibial plateau fractures and ACL tears in both knees. Hall got back to training on trampolines during summer 2010.

His attempt at a fourth gold in slopestyle was upset by Charles Gagnier and in 2009 a fourth straight superpipe gold was snatched from him by Xavier Bertoni.