Thermal underwear is made of a fabric that is designed to keep you warm in cold weather. Thermal underwear has long sleeves and full-length legs. It can be one or two pieces, and is worn under your clothes for extra warmth. It is sometimes called long underwear or ski underwear.
One-piece long underwear, or the bottoms only, can also be called long johns.
Old-fashioned traditional long underwear is red, and has a “trap door” in the back. Sometimes
See the STORY example: I was wearing thermal underwear.