The term 18-wheeler comes from the number of tires on large semi-trailer trucks. It is used as a casual name for semi-trailer trucks, which typically have a total of 18 wheels spread across five axles.
Conventional 18-wheelers are semi-trailers that have two wheels on the front tractor unit and 16 spread across the axles on the trailer. The trailer part is typically configured with dual wheels at the ends of four axles, with one pair of axles on each end of the trailer.
Alexander Winton invented the semi-trailer truck in 1898. This first model had a four-wheel design. The term 18-wheeler arose later, as 18-wheeled trucks became commonplace. These trucks are widely used for transferring freight.