Thermal underwear refers to undergarments designed to retain body heat in cold temperatures. Thermal underwear is available in a variety of styles including the one-piece suit, also called union suit; the two-piece long underwear, called long johns, and the more modern compression tights in separate tops and bottoms. Thermal underwear is typically made of cotton, flannel, or cotton-polyester blend fabric with a box-weave texture. Thermal tights are generally made of synthetic fibers that wick moisture away from the skin while providing thermal insulation.
About Tops:
Tops are an item of apparel that refers to all garments that are worn to cover the torso. Tops may, or may not include sleeves to cover the arms.