Halterneck

Halterneck is a style of women's clothing with a single strap around the back of the neck. This can refer to either a dress or a separate shirt, called a halter top. The latter is a type of sleeveless shirt similar to a tank top (by the American English definition).

A woman wearing a halter top

If a bra is worn with a haltertop, it is generally either strapless or of halterneck construction itself, as to avoid exposing the back straps of a typical bra. Some garments with halternecks may tie at the back of the neck, as is often seen in bathing suits.
In another model of the halter top, there is only a narrow strap behind the neck and a narrow strap behind the middle of the back, so that it is mostly backless. This design resembles many bikini tops, although it covers the chest more and may cover some or all of the abdomen at the front.
Halterneck garments are generally considered more provocative because they leave the upper or upper and middle back free, exposing the shoulder blades. As such, some institutions that otherwise permit casual attire (such as many American high schools) prohibit the wearing of these garments (along with spaghetti-strapped and other revealing clothing).