An upper garment where a closed loop (usually a circular ring, presumably made of hard material) figures prominently. E.g. a type of bikini top where the breast cups are connected in front via a closed, O-shaped loop, presumably of strong plastic or (rust-resistant) metal.
Also applies where an O-ring figures prominently in a bikini top, but elsewhere than between the breast cups (e.g. post #613389, connecting cups to straps or straps to straps).
For the time being, "o-ring" also covers all other loop shapes, such as ovals, hearts, rectangles or any thing else containing a hole through which parts of a garment can pass and then connect together.