A mid layer is an insulated jacket or fleece designed to keep you warm and toasty. The term itself refers to the way the jacket is worn in relation to other garments in a layering system. A “base layer” comes first, resting directly against your skin. Then, one or more mid layers provide warmth through their insulating ability. Lastly, when the weather turns, an “outer layer” or “shell” adds the necessary wind- and water-resistance to keep your entire ensemble dry.