A tractor unit, also known as a truck unit, lorry unit, power unit, prime mover, ten-wheeler, semi-tractor, semi-truck, semi-lorry, tractor cab, truck cab, lorry cab, big rig tractor, big rig truck or big rig lorry or simply a tractor, truck, lorry, semi, big rig or rig, is a characteristically heavy-duty towing engine that provides motive power for hauling a towed or trailered load. These fall into two categories: heavy- and medium-duty military and commercial rear-wheel-drive semi-tractors used for hauling semi-trailers, and very heavy-duty typically off-road-capable, often 6×6, military and commercial tractor units, including ballast tractors.

A trailer is an unpowered vehicle towed by a powered vehicle. It is commonly used for the transport of goods and materials.