What is "thermal underwear"?

Thermal underwear is a type of undergarment designed to provide extra warmth during cold weather. It is typically made of a lightweight and breathable fabric that helps to retain body heat and keep the wearer comfortable in frigid temperatures. Thermal underwear comes in a variety of styles, including long-sleeve tops, leggings, and one-piece suits. Some types of thermal underwear are made with moisture-wicking technology to keep the wearer dry and prevent them from getting chilled from sweat. This type of undergarment is commonly worn for outdoor activities such as skiing, snowboarding, and hiking.