This character wears a chemise.
A chemise is typically a loose-fitting, sleeveless undergarment or type of lingerie which is unfitted at the waist. It can also refer to a short, sleeveless dress that hangs straight from the shoulders and fits loosely at the waist. As lingerie, a chemise is similar to a baby-doll, but baby-dolls are looser at the hips.
NSFW Chemise, NSFW Chemise
NSFW Garter Belt Chemise
NSFW Comparison - Left: chemise, center: teddy, right: baby-doll. Only the teddy wraps around the crotch, and the baby-doll is much looser around the hips than the chemise.