Long johns and thermals are often used interchangeably to refer to a type of base layer worn for warmth in cold weather. However, some people differentiate between the two by using "long johns" to describe a two-piece set of thermal underwear that covers the legs and torso, while "thermals" may refer to any type of base layer designed for warmth, including tops, bottoms, and one-piece suits. In general, both long johns and thermals are made from materials like wool, synthetic fibers, or a blend of materials that provide insulation and moisture-wicking properties. They are typically worn as a base layer under other clothing to provide an extra layer of warmth. However, the terminology can vary depending on the region and individual preferences.