Help identifying this rear-wheel part (photo) I took off my rear wheel, and this fell out on the floor:



Is this a "rear wheel axle"?

It looks like it should indeed be in one piece instead of two. I would like to order a replacement for this part(s) without having to replace the entire rear wheel.
It is indeed a broken rear axle.
If you nave no proper tools (freewheel/cassette remover, cone wrench 15mm, wrench 17mm, new QR axle, 2 rings of new bearings), get it to an LBS to replace it.
It's also recommended to check the wheel bearings cups for defects, it maybe a good time to replace the wheel - all depends on the shape of other parts, and the cost to bring it to proper state.