If a dress is so see-through that light shines through it revealing the goods beneath, it's diaphanous. Also known as "sheer," "transparent," or just plain "sexy," but diaphanous is so much classier.
If you want a classic example of diaphanous clothing check out all those nineteenth century Romantic paintings of voluptuous Goddesses clad in clearly insufficient lightweight gowns flouncing around in the middle of forests at night or storm-tossed fields. Those gowns are diaphanous all right, but because it's a classical allusion there's obviously nothing naughty about it. From the ancient Greek word diaphanes, meaning "transparent" — a style the Greeks were much in favor of.
