Tractor Unit : The part of the machine comprising the engine, driving wheels and driver's compartment that is connected to the other parts of the machine by a gooseneck or universal joint, or a drawbar
Ministry of Business, Innovation and Employment, Glossary of terms and acronyms, Wellington, New Zealand 2010. http://wwwsosh.govt.nz/resources/glossary-u.shtml, (Occupn Hlth and Sfty - Brit) {530}