.A Balaclava is also called a Ski mask
Originally a knitted wool item they are now made of any stretchy material that will keep you warm and make the Balaclava fit snugly to the head and neck
Designed to cover the whole head and neck, exposing only eyes, nose and mouth (most of the time) it's designed for maximum warmth in cold conditions
The name Balaclava comes from the battle of Balaclava which took place in the Crimean war in 1854. They were never actually worn in the war as, despite the need by British troop fighting in the bitter cold, the headgear never arrived in time. Still, the name stuck.