What is a semi-trailer, please?

dxb, it's a big truck (lorry?) used to transport big shipments of goods over long distances. Here's a pic: http://www.man.de/unternehmen/bag_nutz1_e.html. The defining quality of a semi-trailer is that the front part (the rig) can be connected to whatever sort of trailer it needs to haul whatever it is hauling. We may have broached on this before. So what do you call the thing in the picture? A USn term is "tractor trailer" I believe.