An 18-wheeler, also known as a semi-TRUCK or a tractor-trailer, is a large TRUCK used for transporting goods over long distances. It is composed of a tractor unit, which is the engine and cab, and a trailer, which is the cargo-carrying portion. The term '18-wheeler' comes from the fact that the combination of the tractor and trailer has 18 wheels. The crossword solutions for this term are RIG (3 letters) and TRUCK (5 letters).