halterneck

A single strap or material which runs from the front of the garment around the back of the wearer's neck, leaving most of the back uncovered, often used in swimsuits and women's dresses.

Halterneck or strapless!

A halter top or halterneck top (a garment with such a strap).

[…] wearing halternecks, skimpy T-shirts, miniskirts and the like will attract gropers, and the disapproval of both sexes.

[…] she chose her most comfortable skirt and new blue halterneck, packed everything else, then went down to check out before she headed off to explore Toulouse.