From The Art and Popular Culture Encyclopedia
A slip is a woman's undergarment worn beneath a dress or skirt to help it hang smoothly and to prevent chafing of the skin from coarse fabrics such as wool. Slips are also worn for warmth, and to protect fine fabrics from perspiration.
Slips are usually made of a smooth and slippery fabric such as silk, satin, polyester, tricot or nylon, although cotton slips are also used particularly in hot countries. They are often decorated with lace at the edges and hem, and are typically worn over more intimate undergarments. Slips are also worn for modesty under translucent outer garments. A half slip may be worn with a matching camisole as an alternative to a full slip.
A slip dress is a dress for street wear styled like a slip, fitting close to the body and having narrow shoulder straps.