Long Johns are a type of thermal undergarment that is worn to keep the body warm during cold weather. The term "Long Johns" is often used to refer to the specific brand of thermal underwear, but there are many other terms used to describe this type of garment. Some synonyms for long Johns include thermal underwear, base layer, long underwear, thermal pants, and thermal bottoms. Other terms that are commonly used to refer to thermal underwear include long johns with a flap, union suit, and long johns with a trap door. Whatever the term used, the purpose of these undergarments is to keep the wearer warm and comfortable during cold weather.