A semi-trailer truck or tractor-trailer (colloquially known as a transfer truck, 18-wheeler, semi, Diesel, Mack truck (although this is actually just one brand of truck maker), or big rig in the U.S., and transport in Canada; as a semi in Australia or New Zealand; and as an articulated lorry (artic) in the UK and Ireland) is an articulated truck or lorry consisting of a towing engine (tractor in the U.S., prime mover in Australia, and truck in the UK, Canada and New Zealand), and a semi-trailer (plus possible additional trailers) that carries the freight.

In the UK, the term juggernaut is sometimes used for articulated lorries.