A Nightdress (also called a nightgown) is a loosely hanging item of nightwear nowadays mostly for women. Its length may vary from hip-length (babydoll) to floor-length (negligee) but is typically knee-length.
A nightgown is manufactured of e.g. cotton, silk, satin, or nylon. It may be decorated with lace appliques or embroidery at cups and hem. Optionally, panties (UK - knickers) are worn under the nightgown (nightdress).
A short nightgown is often called nightie. A slip nightgown may be used as a nightgown or as a full slip. A shortie nightgown is a functional short nightdress (often just a long or short T-shirt).
A nightgown is often worn with a matching outer garment, a robe, sheer chiffon peignoir or dressing gown.