Tractor-trailer in the context of "Box truck"

A tractor unit, also known as a truck unit, lorry unit, power unit, prime mover, ten-wheeler, semi-tractor, semi-truck, semi-lorry, 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. The largest such vehicles are similar to locomotives. 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.

It should not be confused with a tractor-trailer which is a combination of a tractor unit and semi-trailer, whereas a tractor unit describes only the tractor portion.

Towing is coupling two or more objects together so that they may be pulled by a designated power source or sources. The towing source may be a motorized land vehicle, vessel, animal, or human, and the load being anything that can be pulled. These may be joined by a chain, rope, bar, hitch, three-point, fifth wheel, coupling, drawbar, integrated platform, or other means of keeping the objects together while in motion.

Towing may be as simple as a tractor pulling a tree stump. The most familiar form is the transport of disabled or otherwise indisposed vehicles by a tow truck or "wrecker". Other familiar forms are the tractor-trailer combination, and cargo or leisure vehicles coupled via ball or pintle and gudgeon trailer hitches to smaller trucks and cars. In the opposite extreme are extremely heavy duty tank recovery vehicles, and enormous ballast tractors involved in heavy hauling towing loads stretching into the millions of pounds.