Medieval Fancy Dress
Audrey wears this outfit to attend a maquerade ball at the Eiffel Tower in the film-within-a-film. Her clean, minimalistic costume forms a graceful counterpoint to the outfits of the other ballgoers, which tend to be more colourful and over-the-top.
The hat looks like a silk over a padded stiff base and ties on under the chin:
From the front. Large V neck collar in what looks like black velvet. The white dress fabric looks like a damask. The bodice is fitted, with long sleeves which come to a point at the back of the hand. The high waist peaks at the centre front: