Long underwear, also called long johns or thermal underwear, is underwear with long legs and long sleeves that is normally worn during cold weather. It is commonly worn by people under their clothes in colder climates. In the United States, it is usually made from a cotton or cotton-polyester-blend fabric with a waffle weave texture, although some varieties are also made from flannel, particularly the union suit, while many newer varieties are made from polyester, such as the Capilene trade name.

Source: Wikipedia