A semi-trailer is a trailer without a front axle. In the United States, the term is also used to refer to the combination of a truck and a semi-trailer, also known as a tractor-trailer.

A large proportion of the semi-trailer’s weight is supported by a road tractor or by a detachable front axle assembly known as a dolly, or the tail of another trailer. The rest of the semi-trailer’s weight is semi-supported by its own wheels, which are typically situated close to the rear of the semi-trailer.

Semi-trailers are typically used for transporting goods and materials over long distances. The design of a semi-trailer allows it to carry a significant amount of cargo, and it’s an essential piece of equipment in industries such as logistics, construction, manufacturing, and retail. Semi-trailers come in various types, each designed for a specific type of cargo, such as flatbeds, tankers, refrigerated trailers, and dry vans.