Backless dress
From wipipedia.org
A backless dress is one cut so low at the back that the wearer's back is completely exposed. It may even reveal rear cleavage. It may also have a halterneck for extra support. The idea is to keep most of your front covered to look classy, while revealing enough skin to look daring and sexy.
Obviously, if a normal bra is worn, the rear strap will be visible. To get around this, it may be worn without a bra, or with a specially designed low-back bra or an adhesive bra.