noun - Long johns or long underwear or thermal underwear, is a style of two-piece underwear with long legs and long sleeves that is normally worn during cold weather. It is commonly worn by people under their clothes in cold countries or by cold water swimmers after their swims.
In the United States, it is usually made from a cotton or cotton-polyester-blend fabric with a box-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. European manufacturers use wool blends or even 100% wool, usually Merino or other high-quality wool. Some models might include a thin layer of polyester to transport moisture away from the skin. Wool, in addition to being fire retardant, provides highly effective insulation and will keep its insulating properties even when wet, as opposed to cotton.