The proper term for something worn to bed is called a "nightgown". A "slip" or "chemise" is worn under regular clothes. A "halfslip" is from waist down, and can be of any length. If made of heavy enough fabric (not to see through) such as silk charmeuse, a slip can be worn as a dress to a party by wild young things who like to raise eyebrows. A "dressing gown" or "negligee" usually also has a matching robe. These are worn by ladies in the morning while preparing their toilette, having breakfast in bed, reading the newspaper, and otherwise being extremely casual. Occasionally gentlemen visitors will be entertained while the lady is "en negligee". If you are ordering something slinky for a woman to wear to bed, ask for a nightgown or negligee, and go for silk. It is affordable, and the feel of real silk (from both the inside and the outside) will make the gift extra special. Carolly