I really don't know what to call it, but really it's just a piece of fabric sewn into part of a tube, therefore creating armholes. Yeah...that's about it.
It hangs very loosely and there's no top or bottom...but I think about adding a little button somewhere in the middle. Just never got around to it.

I did however hem the edges as this fabric that I chose does fray, though I didn't hem the armholes....perhaps out of laziness....?
So for something light in the summer it works.