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.