A very large goods truck made of two joined parts: a short powered front unit with the engine and the driver's cab (the tractor), and a long container or box behind it carrying the load (the trailer). The two are hitched together, so the back part can be unhooked and left at a warehouse while the front unit drives off and picks up a different load — that flexibility is the whole point of the design. This is the standard American term; it is the word you will hear on US news reports about motorway accidents. British English says 'articulated lorry' or, in speech, 'artic'; other American speakers say 'semi', 'semi-truck', 'eighteen-wheeler', or 'big rig', and those all point at the same vehicle. It is fairly formal and written for an American word — a police report or a news bulletin says 'tractor-trailer', while a driver in a diner is more likely to say 'rig' or 'semi'.