It seems like women's clothing these days come in one of two basic styles: either formless and floppy like a tent, or skintight everywhere leaving nothing to the imagination. That is such a classic, wonderful cut.

According to my WW2 British Army uniform reference, the buttoned strap at the back of greatcoats is called a "half-belt". Can't imagine it would be something different on any other item of clothing. Makes sense, as it does the same thing as a regular belt, adjusting the tension and fit of the garment.