A tractor unit is a vehicle that is used to pull a trailer. It is also known as a prime mover, semi, or rig. A semi-tractor is another synonym for a tractor unit and is commonly used in the United States. Besides, an articulated lorry in the UK, an 18-wheeler in America, and a truck and trailer are other synonyms for tractor units. A fifth wheel, a dolly, or converter dollies are additional words to describe the hitch mechanism that connects the trailer to the tractor unit. These synonyms are used interchangeably in the transportation industry across the world.