A jacket base is the bottom layer of a jacket that provides insulation and warmth. It is typically made of a thick, warm material such as fleece, wool, or down. The jacket base can be either a separate layer that is worn under the jacket, or it can be integrated into the jacket itself.
The jacket base helps to trap heat and keep the wearer warm. It also helps to wick away moisture, so that the wearer stays dry and comfortable. The jacket base is an important part of any cold-weather wardrobe.