watteau back

from The Century Dictionary.

• noun In dressmaking, an arrangement of the back of a woman's dress in which broad folds or plaits hang from the neck to the bottom of the skirt without interruption; by extension, any loose back to a dress, not girded at the waist. See cut under sack.

from the GNU version of the Collaborative International Dictionary of English.

• The back of a woman's gown in which one or more very broad folds are carried from the neck to the floor without being held in at the waist, while the front and sides of the gown are shaped to the person and have a belt or its equivalent.