A
tractor unit (
prime mover or
traction unit) is a characteristically heavy-duty towing engine that provides motive power for hauling a
towed or
trailered load. These fall into two categories: very heavy-duty typically
off-road capable, often
6×6, military and
commercial tractor units, including
ballast tractors, and heavy and medium duty military and commercial
rear-wheel drive "semi tractors" used for hauling
semi-trailers.