Thermal underwear is a type of clothing designed to provide warmth in cold weather. Made from materials like polyester or wool, it traps body heat while allowing moisture to escape. This makes it ideal for outdoor activities, such as skiing or hiking, where staying warm and dry is essential. Typically worn as a base layer, thermal underwear comes in various styles, including tops and bottoms. It is often lightweight and snug-fitting, ensuring comfort under other clothing. Many people use thermal underwear during winter months or in chilly environments to maintain body temperature and enhance overall comfort.