The layer of clothing closest to your skin. A good base layer will regulate your body temperature through physical stops and starts. It'll keep you cool while you hoof up steep switchbacks, and warm when you break for lunch on a windy ridge. It will pull sweat away from your skin, push it through the fabric, and release it into the air. Base layers should be snug for maximum effectiveness. If there's air between your skin and base layer, sweat will stay on your skin until either cold air finds its way in and dries the sweat, leaving you chilled, or you keep sweating until you are literally soaking wet. See lightweight fabrics, midweight fabrics, heavyweight fabrics, microfleece.
The layer of outdoor clothing used next to the skin. Base layers are normally close fitting, thin, and highly breathable, with insulating and/or moisture wicking properties depending on the need.